Stretch Marks? A rash?

I’m an 18 year old girl, Im 145 pounds, I have large thighs but I’m not particularly “overweight”, I have this on the inside of my LEFT thigh, there’s a little bit on the right thigh but not much. Sometimes it’s way more red and noticeable than others. It looks like stretch marks to me, but I’m not sure why it gets worse and then better if it’s stretch marks? Thoughts?