The symptoms were very warm red & itchy...benadryl (diphenhydramine) alleviated symptoms?

Allergy?: Warm red itchy where? How long? Any infections? Other symptoms? These are a l factors to determine what caused this rash. You can temporarily continue Benadryl (diphenhydramine) amd use Hydrocortisone cream. If it perssits or gets worse see primary or dermatologist.

Dermatitis: Maybe it is hives, which is why the Benadryl (diphenhydramine) helped. I would suggest trying an otc non-sedating antihistamine and switching the Benadryl (diphenhydramine) to bedtime because of the drowsy effects. You might also best be served to see an allergist or dermatologist.