Painful, Itchy, Armpit Rash. Tenderness And Swelling. Taken Antihistamine

what is the best cure for armpit rash that is painful, itchy and due to the itchiness, I have scratched the armpits and now they are tender, swollen and I can't seem to keep them completely dry. I haven't changed laundry detergents, soap, etc. and I use certain dry deodorant. the onset of the cycle seems to begin when the weather changes dramatically from hot humid to cooler temperatures. I don't wear tight fitting clothing either. I have been trying to apply warm compresses in the shower and apply talc powder to try to keep them dried out, but I'm having a hard time to break the vicious cycle of itching, then scratching, then they feel raw, tight and tender. I have tried an over the counter antihistamine to control the itchiness which seems to help, but they are still raw, tender and sore.