My guess would be something called a "hot spot" and it is quite common with cats. My vet usually gives my cat a shot and cream to apply. Miley has had a couple and eventually they go away.

I would take the kitty to a vet, for sure, and get a proper opinion.

Good luck and keep us posted.

P. S. I think it could be the flea treatment.... but unless it is in the spot where you applied it, I would think not..... Just a guess now, don't take my word as the answer! Off to the vet with you!