Hi, I'm Dr. Deb. I will do my best to assist you today.
Benadryl is an ok drug to give to dogs for itching secondary to possible allergies. The dose for a 10 lb dog would be 25 mg 1/2 tablet twice daily or 1 tsp of the liquid kind (12.5mg/5mg or 1 tsp). Side effects are primarily sedation so if she is really groggy, you can reduce the dose to 1/4 tablet twice daily.
Anti-histimines are often not strong enough to control allergic reactions in dogs, however, and they would need something stronger like steroids which a vet would have to prescribe or give to her..
Oatmeal shampoos can also help soothe the skin.
It would be worthwhile to try the benadryl to see if it will give her some relief, however. I see no downside.
Be sure to check Coco over for fleas as well, especially if she is particularly itchy near her tail base or the backs of her legs. She could be allergic to the flea saliva and would itch after they bite her.
Benadryl could also be tried for this particular problem, too.
I hope this helps; if you have additional questions, please let me know.