What Causes A Burning Red Spot Under My Left Arm?

I have a silver dollar sized red spot under my left arm. It doesn't itch but sometimes kind of burns. There are no lumps or bumps. I do shave once a week, and this all started about 3 weeks ago. I have tried lamisil and thought it was getting lighter, then day before yesterday it seems to get worse. Today I notice a smaller area starting in the right armpit also, same thing, no itch but burns. The same razor is used other places and no ill effect anywhere but underarm. I do suffer from seasonal allergies really bad and I'm suffering with that as well right now, with constant sneezing etc. I take claritin for that and have for several years. No new medications, no new deoderant, nothing really has changed just this coming up about 3 weeks ago. Any ideas?