Sternum and mid back pain

For about the past few weeks I have had mid back pain and sternum pain only when I flex my muscles. I do not feel pain when I am really not moving it is only when I start to flex when I feel this dull achy pain. it is not a severe pain just persistant. I know that I really don't rest my muscles other then when I sleep but even when I sleep I always toss and turn. I believe that I pulled muscles but I just dont know why I still feel sore. I am 20 years old and I do smoke. Please help. Tom

[Kalvin]

There are several things that could be causing this pain, one is Costochondritis which is the inflammation of the rib cartilage, and another is Intercostal Neuritis which is the inflammation of the nerves between the ribs. Or it could be rib problem; sometimes they will actually pop out of place. Either way, you should see a Doctor and in the mean time take an NSAIDs like ibuprofen.