Chest pain help im confused?

I'm confused and need help I have been having chest pain and tightness for about 6 months. I have been to the doctor and have been given a chest x ray and e.k.g. test and everything came back fine. I'm wondering if its my body size I used to lift weights and had a good size of mass and I stoped and lost like 30 pounds over the last year. I'm wondering if now when I do strenous things like pushing and pulling I get chest tightness because my chest has lost mass and its to tight or something? Can anyone help me please thank you. These pains are happening now just when I'm sitting down?