I would have this looked at. Worst case would be something like cutaneous/dermal hemangiosarcoma (pitt bulls are susceptible), but I hope that it's just something minor: http://www.danhonovich.com/p145624857/h97697D3#h97697d3
(the description that goes along with that picture isn't a very good one for the dermal version).
I would also want to have a blood count run, to check on platelets - if they are low, it can cause bleeding into the skin, though it doesn't usually look quite like that. Low platelets can be from tick disease, vaccine reactions, etc.
Let us know what the vet says!