Should I be worried about my dog's golf-ball sized mystery?

I just recently arrived back home from a week long vacation, while I was gone, my dog stayed with family members(he has stayed with them in the past, they take excellent care of him so I'm not worried that this was cause by abuse). The morning after he was returned to me, I noticed that he was scratching excessively, the kind where when I scratched him, his leg would start t on the ground, I checked for fleas/ticks and didn't find any. Since the family he was staying with has a cat(and because he was in a different neighborhood outside), I decided I would give him a flea bath, just in case. During the bath, I found a golf-ball sized area on his bottom that was large and swollen. He moves away from pressure, but pushes his bottom towards me when I touch other parts near his tail. It feels like a big, squishy bruise. Should I be more worried than I already am? He is a 11 year-old Yorkie.