Since you have got all basic investigations done and evaluated your heart and lungs as well that leaves you with a muscular cause for the tension of muscles of the chest.
There can be two causes for muscle tension, either physical or psychological/emotional. In the former, usually, injury
, strain, overworking a muscle, repetitive motion, and the use of poor body mechanics while work outs can all cause muscle tension. And psychological causes are anxiety
, low mood or more complicated ones that one might not be aware of him/herself.
For a physical problem you may visit an orthopaedician or a sports medicine
specialist if available and for a psychological one, a psychiatrist
can help you out.
Hope this helps,
Dr. A. Rao. Kavoor