Underarm Painful Irritation

Hi,

About 2 days ago I noticed a small red spot in my underarm, it was a little painful but I didn't pay much attention to it. I suspect it may have been the strong anti-perspirant I use but I'm not sure, I've been using it for over a month and this is the first time I get an irritation.
Overnight, however, this thing has grown and yesterday when I woke up it was extremely painful, and my whole underarm is red, extremely dry, and sort of cracked at certain spots. It's only on one underarm, not the other, which is strange.

This has happened once before (or something like it) after a 12 hour plane trip, about 3 or 4 years ago.

I've been applying aloe vera moisturizer a few times a day to see if it helps any, but so far no signs of improvement.

What can I do??

Thanks