My cat Morrison has chronic allergies and I have to be hypervigilant against fleas. Even when they are treated it's possible for them to be brought in from outside. Though I see you are in PA so that's probably less likely there this time of year than it is in Louisiana where it's a year round problem, but as others said it does only take 1 flea.
If you think it might be behavioral, there are calming supplements that you can get OTC just to see if they might help. When my vet first suggested that maybe we might try an anti-anxiety medication for Morrison, I tried a supplement called Pet Eze while we were waiting on the dermatologist to get back to her with her recommendations, just to see if maybe we were on to something. Morrison was completely miserable and even though he was on cyclosporine and we had upped his dose he was tearing himself to shreds, even with an e-collar. The active ingredient is l-tryptophan and I noticed an immediate effect. He mellowed right out and wasn't scratching himself constantly anymore. He's not taking Pet Eze anymore because we've since put him on rx doxepin, but it bought us and him some time until we heard back from the dermatologist. I'm fortunate that Morrison is very easy to pill but there are similar products in treat and paw paste form.
What flea preventative are you using? According to my vet, some are better than others in reducing flea allergens. Advantage and Revolution are both good according to her. I can tell when Morrison's flea preventative has worn off because he IMMEDIATELY starts itching again.
Another thing to look into that we discovered with Morrison is the possibility of a yeast infection. He hadn't been tested for it since we were trying to dx his allergies, but my vet cultured his fur about 6 or so weeks ago and he was badly infected. He got an oral medication to take care of the yeast and it's greatly improved. He's now getting weekly baths to keep it from coming back. He is so much better now than he's been in several years. Allergies are tough.
