I Have Old Stretch Marks On My Bottom From When

I have old stretch marks on my bottom from when I was a teenager (I'm 26 now so theyve gone silver in colour and hardly noticable). The other morning I woke up to find a small round patch of them look very dark in colour and bruised looking but only over the stretch marks and not the skin in between. Could this be a bruise? Why is the effected area only on the stretch marks?