Inflammation in armpit?

This just started yesterday, with an annoying itch in my right armpit area. It's not exactly IN my armpit, but very close to the pit itself, right on the front side. I thought it was maybe just a bug bite, and was itchy, so I tried to ignore it for the rest of the night. When I got home, I applied some benedryl anti-itch stuff to it. However, when I looked in a mirror, I couldn't see any actual 'bite', but I figured it was just because I had been itching it and it was red.

This morning, it's been waking me up in the night with the itchiness. My armpit area is very sore and tender, and I have to sleep with a blanket between the area so the skin doesn't rub against it. It doesn't look swollen compared to the other side, but it's red and quite hard when I press on it. (The other side doesn't feel hard). There is no rash or breakout or anything like that. It's just red, painful, itchy and hard to the touch.

Anyone have any clues as to what this could be?