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

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.
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.