You should have this checked out to be sure it's nothing serious, but I will tell you my story.
I have seen a couple of doctors for the same type of sypmtoms. My pain is hard to tell if it's the breast or chest wall pain. I've been told it could be costochondritis.(see the post on Lupus board). This can be aggravated by heavy lifting or similar activity. I feel much better with motrin or advil.

I also have more swelling/pms pain in my left breast some months. I've always had normal mamograms - last one in June, and I have had breast exams by my gyno in between - last one just yesterday. I am about 1 week from my period and have felt some pain and swelling the past week.
I'm also 48 years old, and am experiencing alot of peri-menopausal sypmtoms, of which this is probaby one.
It's not always usual to have the symptoms in mostly one breast, but not impossible either.
Another factor in my situation, is that I had a breast reduction about 15 years ago - according to Dr., this may make my breasts act differently. When tissue is removed, the surgeon is not necessarily removing the same amount of fat and/or breast tissue on each side, so my left breast may have more sensitive breast tissue than my right.

Hope this helps a bit. If you feel better when you get your period, it's probably hormonal - but get checked out for your own peace of mind.

Mary