What Could Cause Itchy Rashes And Tiny Red Bumps On The Upper Arm?

i have had a rash on my left arm for weeks. It comes and goes and is on the upper arm underneath (not the arm pit) in the soft flabby part and a little on the forearm where I had a mosquito bite. Weather has been really hot but the rash is no where else on my body except a tiny bit on the right arm same place (underside). It itches really bad and has a few little blisters. It has a bunch of tiny red bumps. Im putting Benadryl cream on it every few hours. no new soaps, lotions, detergents, etc. No unusual interactions with plants