Does my dog have allergies?

I have a mastiff/rottweiler mix and she is going on 3. For some reason this summer I have noticed her scratching and biting a lot..I do not see ANY signs of fleas what so ever and I also have 2 others in the house that have no signs of itching. I give her benadryl daily and this doesnt seem to help..I plan on calling the vet but thought I would ask all you pet lovers first..Any ideas?