17 years old

I've been focusing more intensely on lifting since may, and even after 2 years of not-so-regular training, I managed to gain around 12 lbs while keeping my stomach somehow flat (I'm a hardgainer so I don't accumulate too much fat while bulking).

By doing so, I also managed to get A LOT of stretch marks on my biceps and a few over my shoulder, and when I rub my hand over them, my skin feels (idk if it is but it feels like it) fragile and sensitive.

And the fact that I acquired all of them whithin a few months worries me cuz I'm not big yet.

Anybody experienced it, and how do you slow down this process? Do I miss nutrients or something? Should I take a break from bulking/going to the gym? Or I shouldn't worry about it?