Ok...it could be a lot of things, but I have a TON of experience with this. I have a kitty that has HORRIBLE AWFUL NO GOOD flea allergies.
We used to have a REALLY hard time getting them under control. We wouldn't see any fleas for weeks, none on any of the pets (3 cats, 1 or 2 dogs at any time) nobody itching but Kyle. (the cat) Oh, and everyone would always be Frontlined or Revolutioned. It turns out that even ONE LITTLE STINKING FLEA in the house would be enough to send him into an itching frenzy. Even if we couldn't see any fleas. Even if it was the last lowley flea in the house.
Fleas are HORRIBLE to get under control and even when you have eradicated all of them, there may be ONE egg left under a couch somewhere that can hatch, bite ONCE and then die. But, that one bite can put the itch-scratch cycle in motion.
For Kyle, if he STARTS itching, it makes him scratch, which makes him itchy which makes him scratch...at some point, even long after that last little flea is gone, he's still itchy and scratchy.
I know that you think that you have all the fleas gone, and you might, but you also might want to just continue to do whatever you did to get them under control. Just treating the animal with Frontline is NOT enough. YOu have to vacuum like crazy, use Diatomaceous Earth, whatever you can do to also treat your house.
If its not that, I don't know. Its just been my experience. Also, if he seems to stop scratching for four hours after you give him benadryl, I would say that that would DEFINITELY indicate an allergy! Its fine while its active but bad when it wears off? Why would you think that ISN'T an allergy? You have to get him to stop scratching, so give him the max. dose of benadryl at night and do whatever you can to get your flea problem under control completely.
Just my suggestions!!!!!
WElcome to BPO! I'm Nicole!