Thanks for that info. My first impression is that this could be a bacterial infection, such as staph. To diagnose this your vet will need to do a scraping or biopsy to confirm. This will require antibiotics to clear up; possibly a long term treatment.
Food allergy is always a possibility - it can happen without a food change and the antihistamines won't touch this problem. I would try a hypoallergenic food with a "unique" protein - something she has never had before, like duck or venison. It is imperative that you do not give table scraps or treats, just the unique food, in order for it to work. Allergy testing can be done by a dermatologist to find out exactly what your dog is allergic to.
I know you may not see fleas, but dogs can become hypersensitive to fleas, and just the presence of one flea for a short period of time can cause a huge reaction. My own dog is extremely susceptible and he will lose all his hair on his back if even one flea gets on him. It's called flea allergy dermatitis. For this reason I would treat with a flea product monthly to eliminate this possibility.
You may want to try a different antihistamine, such as Benedryl, given at 1 mg per 1 pound of body weight every 12 hours. Different antihistamines have different modes of action, so it's worth trying a different one and seeing how it works.
I really think you need to consider the possibility of a bacterial infection and a course of antibiotics. Discuss this with your vet. Ask him about Simplicef, which is a fantastic antibiotic for skin infections. You want a long term course of this antibiotic, at least 3 weeks. Also ask your vet about Temeril - P, which is a combination of a steroid and antihistamines that is very useful with skin problems.
So I tried to give you some info on all the possibilities, but strongly consider talking to your vet about pursuing treatment for a bacterial infection. I think this is a very likely cause.