I'm 18 curious on why I'm getting stretch marks so fast under my armpits, my hips, under my breasts? I've just noticed that I have two small stretch marks above my vagina like on top. Also I noticed more every time I look at my body. I'm 5'7&1/2 I weigh 190-195lbs. What can cause this to happen ?

I tried to find better words to say this... but a lot of it is weight related. The rest of it is genes (some are born luckier than others). If you are darker complected its going to be easier to see than if you are fair complected.

Ok thank you. Yea I noticed I've been losing and gaining weight. That explains it my body never has been like now. I started with stretch marks on my stomach now its out of control. I'm getting them fast & quick & I'm starting to get one above my vagina is that normal ?

Start moisturizing after every bath or shower. Cocoa butter and Vitamin-E oil will help.
If your weight has been fluctuating then that is a major part of it.
You can get them most anywhere.
It happens to nearly all of us.

I'm going to start doing that more often. Thank you

You pegged it. Weight loss and gain.. however good news... you are young... they can still go away. Cocoa butter is something many women who have babies use to help... its just a bit more extreme than what's going on with you... but really the same reasons.