I got booted off while I was writing this, so plz let me know if my first post somehow went through (I couldn't find it).
In order to answer the post about the underwire bras, I had to register, then I got forwarded to the main page - but I think the initial post was in this thread. The initial poster said she was in great anxiety because she had a lot of pain under one arm. One of the people who answered said she had pain under both arms and thought her pain was because of the underwire bras.
Anyway, I also have a lot of underarm pain. Since it's under both arms and since the doctor didn't feel anything wrong there 2 months ago, I'm also assuming it's from those darn underwires. If you're very short like me, the wires dig into your underarms constantly. It takes years to develop; I didn't notice until it started hurting a lot and I could feel that it was right where the wires dug in. I now take the time to dig for the soft bras, sometimes I have to pay a little more since underwires seem to be the preferred style for D cups, but I hope in a few more months the pain will gradually decrease.
If someone has another theory, I'd like to hear it before my next dr's visit so I can get a more complete checkup if warranted. I didn't get a mammogram this year due to lacking health benefits, but I'll foot the bill if I think something serious is going on.