What Causes Soreness Under My Armpits?

After an 8 mile run this past weekend, I had considerable soreness under my armpits. I wrote this off to a rash from chafing, which I have experienced before. The next morning though, I had a painful lump under both armpits. They seem to be getting a little smaller, but have lasted 2 days now. Is it possible that the sweat glands were infected? What should I do to treat this if so?