It could still be a flea allergy. All it takes is 1 or 2 flea bites to start
a skin reaction which then itchs & spreads the rash & the itches. The
fleas will die because of flea treatments, but still get in a few bites before
they do. Food allergies are also possible. Some dogs are just more allergic
than others to the flea saliva. I hope you can get it stopped.