upper-mid back pain when breathing.

So I read threads about other peoples back problems, but I figured I'll share my experience in a new thread: I've had back problems now for about 3 years. Im 25, and always figured it would just go away after a while like most other pains I've experienced. Unfortunately it never did. My pain seems to stem from deep down under both shoulder blades. When standing, or sitting for long periods of time, the pain gets almost unbearable and I find myself stretching, or, trying to crack my back to alleviate the pain. Sometimes it works, but only temporarily. Also, when I finish working out, or running on the treadmill, and I'm breathing deeply, it hurts terribly. I've been for physio, and nothing and I did massage therapy which seemed to help a little bit, but didn't rid me of the problem. I'm very concerned, as I do a fair deal of physical labour at work. I know this issue will only get worse with age, and I would really like to nip is in the bud... Any thoughts or comments would be greatly appreciated.... thanks!