Dear 1scared girl: It is true that pain is not a usual symptom of breast cancer. It is also quite possible that your use of new muscles, underwire bra and manipulation are contributing to this. It is not uncommon for people who work in health care to become concerned about illnesses, but from what you describe, you have nothing to worry about at present.
I don't want to scare you...I work in the medical profession also and know how scary it can be. However, please do not assume everything is ok just because breast cancer doesn't "usually" cause pain. I had pain in my right breast probably for about 6 months before I had it checked out. I thought it was my underwire bra too, then cysts. I did not initially feel a lump either, so I did not think it was cancer. Afterall, my previous mammograms were negative, and nothing was ever detected on breast exam. But in February, I had a cancerous breast tumor removed, which my surgeon said was probably there 3-5 years. It was very deep, right near my chest wall. Thus, it never showed on a mammogram. This time however, since I actually felt a lump, they kept shoving me into the instrument until it actually showed up...right on the very edge of the film. If I had not felt the lump, it most definately would have been missed again.
I pray you don't have cancer, but if you do, better to stop it early. I hope I haven't scared you to death (I know how I would have felt if I had read this when I had my pain!), but I just keep reading about pain NOT being a usual symptom. From my personal experience, if I had NOT had the pain, my tumor would probably not been detected when it was.
Ask for a breast ultrasound..it could pick it up easier if the location is deeper. Good luck to you.