What you have is called stretch marks, and they are by no means just things pregnant women get!
You should be able to find out plenty if you type 'stretch marks causes' into a seach engine such as google.
They look their worst when they are new and fade to skin colour eventually, though the channels tend to remain, but not nescessarily very obvious!
I once had a friend say how lucky I was to have no stretch marks, (she was giving me an aromatherapy massage at the time) I could only laugh as I am covered in them!
So you see, they do fade