What Causes Greyish Blue Discoloration Under The Navel After A C Section?

I had a cesarean section 2 weeks ago, and developed a greyish blue discoloration under my bellybutton- I thought it might be a bruise, and it is somewhat tender, but I'm not sure. I didn't have anything like this happen with my first c-section 2 years ago.
I am 25, female, 220 lbs, 5'9"