Random stretch marks??

Hey, everyone!

I have an issue that is bothering enormously. About two months ago, I got this giant stretch marks in my belly, on either side of my belly button. I also got a very small one on the side of my right boob. Now, I am not pregnant nor have I experience drastic weight changes. Today, I discovered some on my arm, which are not as dramatic as the ones in my belly but still!!!

I did gain weight last year, about 7 pounds... but it was throughout the year. I have always had a bigger complexion, but I just don't understand this and is making me feel so sad. I am only 20 and I already have this huge marks on my body for no reason.

Do any of you know why this might be happening? How do I make the go away?