What is this hard lump in my armpit?

I am a healthy, 24 year old female and am very hygienic. I intend on going to the doctor tomorrow (or the next day..whenever I can get in) but I'd sort of like to know what I'm facing when I get in there. About a week and a half ago, I found a hard knot/bump on the lower part of my armpit. It was very tender to the touch and flaming red. Exactly one week later, it went away but at the same time another one came but only on the top part of my armpit. This one is so much worse. Flaming blood red, getting bigger, and a very hard knot/bump. Probably half of the size of a golf ball. The area around it is also very red and swollen. It's been there for about 5 days now. Any ideas as to what it could be? Like I said, I have every intention of getting it checked out, I was just wondering if anyone went through (or is going through) the same or similar situation. Thanks in advance!