red lines on my inner thighs and all over my but cheeks?

(I'm a female, 15)

(Not pregnant)

(Never been pregnant)

So I was looking it up online and apparently it has something to do with growing fast or gaining weight. Before I injured my knees, I would run about 4 times a week, I ate like an average person. I was like 5'4 and 118 lb. After 5 months, and my kneels are still healing, I have not ran or exercised at all. I've noticed the inner thigh marks for about a month to two months now (I see them everytime I use the bathroom) but today I turned around and looked at the bathroom mirror and there are A LOT of marks all over my butt cheeks, and the sides of my butt, lower butt, etc. I can even feel some of them. I showed my mom them (I was wearing an underwear) and she got scared and said she wanted to take me to the doctor, something about veins.

I am now 130 lbs. I gained 18 lbs over a period of 5 months because I haven't been working out.

Should I be concerned or do you think they are just stretch marks? These marks are in areas no one's really gonna see (I swim with shorts anyways, bikinis make me uncomfortable), so I don't care that much if they're just stretch marks, although I wouldn't mind treating them to make them show less.

Thank you in advance x