Injecting on stretch marks & is this pen empty?

First question: has anyone injected into an area where they have stretch marks? I’ve been injecting into my arms for a few weeks and there’s usually some pain, nothing horrible, but i definitely feel the shot. Today I did it lower, where I have stretch marks, and it was so painless it was concerning. When I used to inject into my stomach, one side would hurt more than the other. I discovered, the side that did not hurt, also contained stretch marks. Do we know if the stretch mark areas are painless?

Second question: the first question leads to me being nervous about if my pen worked correctly. I’ve had painless injections, but never THIS painless in my arm. This pen is empty correct? The no pain is really causing me to be nervous about not getting my dose correctly LOL

Obviously I don’t like feeling the shot but it lets me know that pen is working and I did it correctly