What Causes Brown Blotches On The Abdomen?

Hello, three days ago, I laid on my bed bar while typing on my computer. When I got up I saw two big red spots on both sides of my abdomen. I did not worry about it as it can happen sometime when I put pressure on some part for more than 2min. But since then those red blotches turned brown and giving the impression of skin discoloration, it is not hurting but I have never see this before, It usually just go away with not marks. Should I be worried?