weird pain/pressure

[nike103103]

Everytime I get serious about liftin I always get this weird pain that starts out as just like a pressure and then gets pretty bad. I have gone to the doc about it and they always say everything is ok. Latley it has been happening just to the right of my left shoulder blade. When I run or jump around, move my neck in a certain way, or stretch my left arm all the way out, or breath in fast and deeply I can feel a small pain in the location I explained. Whenever I get it though it is always in a different place. The fist time it was in my ribs, then last time on my right side, and now this time on my left. Does anyone have any ideas on what it could be? Like I said I have gone to the doctors because of this but they cant ever seem to find anything. Thanks