What Causes Brown Marks On Stomach?

i am 16, i'm 6'1" and i weigh 145. i have these brown marks on my stomach, the marks are only on creases were my fat used to be (i used to weigh 180 and i lost weight, my stomachs flat now but i have those brown marks.) what are they? how do i get rid of them? they are the only thing stoping me from wearing a bathing suit this summer :(