Definitely take Cheeky to the vets.

My Iggi also has a flea bite allergy and is treated for fleas every 4 weeks.
He always flares up in late August/ early September and pulls the hair out just where Cheeky does too (and sometime the tip of his tail and around his neck are affected).

The vet thinks it is an allergy to a particular seed/ pollen as a cat with one allergy is very likely to have others. Iggi does go out but not much however the vet thinks it would be difficult and costly to identify what is causing the allergy and practically impossible to isolate him entirely from a pollen anyway.

Hope this reassures you that it may not be a big problem - but still best to check.