My Staffy has the same thing, last Monday after obedience training she came up with a lump the size of a golf ball on her side just down near her back legs and rib cage, I took her to the vet and they put her on antibiotics and anti inflamatorys but it doesn't seem to be going away so i'm going to take her back down to my vet to get her checked out again, my vet said it was an unknown infection of some kind and should go down but hasn't so i'm thinking it could be something else and she may need surgery
I'm starting to worry as some of my friends dogs down at my obedience school have had to be put down due to cancer but i don't think that's what it is, but we just can't think of what else it could be at this stage. So it's off to the vet this week again to get it checked out.