It CAN still be fleas. Advantage is NOT a repellant. There are no flea repellants out there yet for dogs or cats. So, the fleas can still get on and bite, but they die very fast after they come in contact with treated pets or areas where the pets have laid.
Some pets are extremely sensitive to flea saliva, which the flea injects when it bites. The saliva has a protein that keeps the blood from clotting while the flea takes her meal (it's the females that bite, mostly). This protein causes the itching, same as how a mosquito bite itches us.
Even if it isn't fleas, it can still be allergies. In veterinary medicine, it's common practice to treat allergies symptomatically rather than testing for specific allergens because the testing is quite expensive for many owners. It's even possible to have allergy shots (desensitizing shots) made for pets-- our clinic has a few pets that get monthly shots like this and they do help-- but again, it's rare.
Our vet uses Benadryl a lot for general allergy relief, especially for pets who can't get steroids like prednisone or Depo-Medrol, and even for those who do to get relief at home.
Our vet usually prescribes Benadryl 2-3 times a day (every 8-12 hours) at the rate of one milligram per pound-- thus, a 25 mg. tablet is safe for a 25 pound dog, half a tablet (12.5 mg) is fine for a 10-12 pound dog (or round it up to a 15 pound dog). Cats get the same dosing rate. **THIS IS NOT ADVICE. THIS INFORMATION IS FOR EDUCATION ONLY**
It can make them sleepy, but not as much as it does to us. I use Benadryl to calm my cat for travel or a vet trip. He has a heart condition that precludes other kinds of sedatives, so the Benadryl works well. I'm a vet tech and with the doctor's guidance we give him twice the "allergy" dose. He does fine with it, and he is much easier to handle when we have to do his heart sonograms. I also give him a dose when there are storms in the area because he totally freaks and then his heart starts beating too hard, and he begins to pant. He's on heart medicine, and there is no interference or cross-reaction between the two. **THIS IS NOT ADVICE. THIS INFORMATION IS FOR EDUCATION ONLY**
Talk to your vet about the specifics for your pet based on their medical history, but in general... Benadryl is very safe.