Mysterious Axillary Pain, Clear Mammo/US

I'm under high surveillance routine because I'm BRCA 2 positive, decided not to do prophylactic mastectomy.

I've been suffering with ongoing axillary pain on my left side for at least 4-5 months. Tried ibuprofen heavy duty, does nothing. Pain doesn't seem to be tied to cycles (I'm post menopausal and not on any hormones). Had physical exam, where Dr said it seemed "full feeling" but no real mass or lump identifiable. Had quickie ultrasound that turned up nothing. A couple of months later, pain is still present most of the time, so they just did a mammo on that side which turned up nothing again.

I'm happy that there's no sign of cancer, but has anyone else dealt with this type of mystery pain? It's located behind the fat lump where my arm meets my body. Sometimes it's pretty severe, most of the time just a dull achy feeling like a pulled a muscle. (I haven't lifted anything heavy, and I don't do strenuous work, I work at a computer.)

Any thoughts? Maybe I'm just crazy?