They could be stretch marks. These can occur anywhere on the body where the body has grown or you have put on weight too much too soon.
Some women get these on their upper arms, breast, thighs, stomach, virtually anywhere. They can get it when growing in their teens, pregnancy, putting on weight too much too soon.
The darker your skin the more noticeable they might be. There are creams sold in the chemist to lessen the marks.
