Girlfriends ex-coworkers contacted me to say she cheated on me 3 years ago.

Girlfriend and I have been seeing eachother for 6 years, we're mid/late 20s now looking at buying a house together and kids etc. When we met she was shy and had no real friend group and it's mostly just been me and her since we first met. She never goes out with friends and neither do I, we've always been happy with eachothers company. I was her first boyfriend (also first guy to take her to bed) and its been smooth sailing since day one. She worked in a high end fashion shop 3 years ago for about 6 months and in this time they were brought to a fashion show in Italy. When she came back she left the job only a few weeks later. I Never had any doubts or worries that she would do anything wrong on her trip. Her old coworker (who is a guy) contacted me randomly last week and basically said that my gf and her boss (literally a basic old man, very fat, bald, 5ft6) missed the fashion show and when they all met for dinner afterwards my gf and her boss didn't really have an excuse and they left the dinner early. He said they both had rooms on the same floor of the hotel even though the other workers all stayed on a different floor. When my gf "left" her job her boss had a meeting with all the staff where he told them that he had to fire her because of her refusal to do what he told her and their constant arguing. The coworker said he had never once witnessed them argue and she always did what the boss wanted like everyone else. The coworker in question isn't interested in girls so I knew it wasn't a jealousy thing. I still didn't really believe him and kinda brushed him off with a blunt reply but this morning a female coworker of hers who used to pick her up and drop her off after work messaged me to tell me that everything the first guy said was the truth and they felt they had to let me know when they saw my gfs posts about us looking to buy a house together. I haven't said a word to her yet because everything has been perfect for us up until this and I'm in a state of shock, is there any other reasonable conclusion apart from my early 20s gf cheating on me with a fat loser in his 60s? I'm trying to build up the courage to confront her and I wanna be sure before I make such an accusation.

UPDATE: haven't talked to her about it yet because of my shifts and her shifts I haven't had the time to get into it but I met a good friend of mine (he's close with her family and that's how I met her) and told him about the situation. He was obviously shocked but instantly jumped to cheating also. He told me that there was something that didn't sit right with him that happened with her a year ago aswell but it was so out of character for her and he knew her so well he convinced himself it was nothing. He said that at a family event that I wasn't present at she got very drunk very early in the evening and left the bar, he said she returned an hour and a half later in a cab and said she had to run home. He said he didn't question it any further but when he saw me come pick her up when the party was over he wondered why I hadn't just dropped her back off earlier on. I was at home this whole night on the couch waiting to come pick her up and she definitely didnt shoot home for something. I didn't go because I felt she needed some time to socialise without me always being with her. My friend hinted at the fact he thought she might have left to get drugs but for one it wasn't that type of event, the girlfriend I know would never ever dream of taking drugs nevermind wondering away on her own in a cab infront if everyone to go and get said drugs to return to the same party to take them and plus we both pool our money and we dont have that kinda cash and i would have noticed her taking money at the time because I know her buying a new dress and spending money to go to this party was already a financial strain at the time. Also since my last message and all of the replies I have done some more digging and realised that in her search history was, no joke, adult videos with older guys, she searched it 4 different times in the space of a week and at the same time searched for anal plugs and it says there was a confirmed order. I have never in my life heard her mention toys or the fact she watched any form of porn. I know everyone is gonna say oh but none of this is proof of cheating but at this point I feel like I just hadn't been looking past the fact that everything was perfect without being a little bit more critical of certain things at certain times in our relationship. I think I might have been too enamoured these last 6 years but it feels like it was always in front of me but I just never would have thought to look for it. I'll be having a chat with her tonight but honestly as I said she tells me everything so why not tell me she's got kinks or why not tell me she had to leave the party and go back, we're together 6 years if she had to leave because she had diarrhea or something embarrassing she would tell me I can't see why I don't know this, im afraid to talk to anybody else about it incase they tell me another story, I think this is it for me and her regardless of cheating or whatever because I feel like I don't know this person at all because of these last especially 2 days.

UPDATE: We had a talk and it didn't go well, she was caught out and denied everything, massive argument and I told her I was leaving. So we're finished. She had since been in contact with me and asked if we can try again and maybe we could have an open relationship...so I really didn't know who she was at all

FINAL UPDATE: Lots of you still are saying it could have been SA and that it's still speculation. I was brief in my latest update because it was raw. We started the conversation with me asking her about said co workers and asking if she's still in contact with them and she said yes she's still in contact with them especially the girl who contacted me second. She asked me why and I showed her both messages and she denied everything. She never said anything about assault or trying to cover it up but saying none of it was true at all, that she did attend the fashion show and she didn't leave the dinner early, she said they all roomed on the same floor, so she said there was absolutely no truth to what they had said to me. I knew this wasn't true because one of the coworkers had a Snapchat memory saved of the dinner and of the fashion show, aswell as messages to my ex saying how strange it was that they were on a different floor from everyone. So I told her about all this and she got extremely defensive and tried to dispute the proof I had but had no way to prove her version of events. Then when I brought up her leaving the party in a cab like our mutual friend told me and she said she doesn't remember where she went and once again tried to say she didn't leave the party and that it was a lie. Then she told me the buttplug and the videos she was watching were something she was going to tell me about because she wanted us to try role play and bring toys in the bedroom but these searches weren't recent and as I said the toy said it was delivered but I've never seen it and it isn't with her stuff leading me to believe it's at some guys house. When I left she said she understands why I left even though she didn't admit to a single thing. That's when she said she wants us to get back together under whatever circumstances even if we have an open relationship. Since this conversation she has posted that she is going on a trip to the UK solo, I asked the coworkers and their boss isn't going on the trip so she isn't still having an affair with him, maybe she has another guy or maybe she is just travelling solo I don't know. But I think that's the end of the story, im done with this whole situation and I'm still waiting for it all to hit me and see how I'll deal with the emotions but honestly I feel like it's not real.