Hey
I have these too. But in addition to them I also have some pain associated with them. Now, I have never been overweight but I have a physical job which requires alot of climbing and lifting. Could this be job related? I am afraid it could also be more serious than stretch marks--I think it could be cancer I hope not but I think this is a question for an oncologist..
The groin lines do sound like stretch marks, and the hips are a common place for them too. I'm not sure why you're worrying about them now, but if you see a doctor in person, he or she will be able to tell you that they are, so you can ignore them. It's quite likely that your weight fluctuation had an effect, which may have lengthened them.
If you have other reasons to be fearful of an STD, such as unsafe sex, please avoid it. If not, just get yourself checked out and move on.
Take care.
Dr. Rockoff