I think my cat might have nerve damage ?! ):?

Okay so my cat is 9 years old and a little over weight. Ever since he was a kitten he would do this weird thing where if you scratch his but it would make him lick himself ! But it never really hurt him. Well lately if you even pet him on the back he'll almost attack himself like he's trying to get a flea. and sometimes he cries, but not often. I'm going to take him into the vet this week but I was just wondering if anyone could give me a heads up on what might be going on?