L.R.
You don't mention having ever been to a doctor about this!
Please get a thorough checkup and tell the doctor in detail about this pain.
It could be serious; it could be nothing; but you can't get it diagnosed on Mamapedia. It could be: Heart problem; muscle pain; infection; or a breast problem; or anything. Only a professional can tell. If this has gone on for years please do something about it.
If they have trouble diagnosing it, be sure to see your gynecologist and ask if it could be fibrocystic breast tissue. This condition means benign (not cancerous) lumps and bumps in the breast and can be very painful and frightening. Try cutting out all caffeine and see if that helps; caffeine encourages formation of the benign cysts. Many women have it and go undiagnosed unless they know to ask about it or unless a gynecologist feels a lump in their annual breast exam. But don't just cut the caffeine and wait to see what happens; see a doctor now.