The dose for benedryl is 1-2mg/pound every 8-12hrs (Plumb's veterinary drug handbook). Some dogs don't respond well enough to it and need something stronger, like a steroid. You should not give your dog asprin if you think she might need steroid for comfort. If the pain is that bad the she's not responding to antihistamines, she probably has a secondary skin infection that needs antibiotics. You can try to find an OTC calming shampoo with oatmeal or sometimes low doses of benedryl or cortisone. If she's that bad, a vet visit may be required.
Since your dog has flea allergies you want to prevent them from biting as much as possible. There's two good products out there that will help with this. One is Comfortis, it kills fleas so quickly, they don't bite multiple times. This reduces the total amount of allergens your dog is exposed to. The second product is Vectra 3D (Firstshield from Banfield). It kills and repels fleas, ticks and mosquitos (plus other pests). These two products are only obtainable through vets, but they are better than the other products out there (advantage, frontline, advantix, hartz, etc). Since you live in warm environment, you need to use flea prevention year round. It's easier to prevent infestations than control them and your dog suffers if you don't.