" There's a key change or something in there that's kickin' my but-tocks."

I believe the "key change " is actually a modulation.

The verse is a rather simple tune, perhaps a varient of the one used in "Me and Bobbi McGee" and Steve Goodman sings it in the key of C.

The chorus is in the key of F which is a bit confusing since the F maj is also the IV chord in the verse. It is the tonic (or I) in the chorus.