Struggling with my body changing in my early 20’s

I’m 22F, I have always been pretty small my whole life with a high metabolism (I ate quite literally whatever I wanted and was still skinny). My weight usually hovered around 130 pounds and in the past 6 months I’ve gained at least 20 pounds mainly focused in my thighs, ass and hips. While I was on vacation in the keys I had very bad chaffing between my thighs to the degree of having to use deodorant to help with the friction. I had never had to deal with something like this before. While I know having an ass isn’t a bad thing, I discovered stretch marks on the back of my thighs and I basically had a melt down with the fear that a guy I’m intimate with will think they are gross. I suddenly feel like a cow and seeing people with the body I used to have makes me envious. I’m constantly self conscious of how my stomach looks in jeans when I sit down especially if I wear a crop top. Can someone tell me if my stretch marks are normal and that I’m over reacting??

(I also am bipolar and struggle with manic episodes and mood swings)