Hi harry,
I'd say this is possibly part of the anxiety and panic attacks. When you are stressed you will be subconsciously tensing the muscles in your diaphragm and chest.
Go to the doctors anyway and just say exactly what you have said on here. Even if it not there all the time. Doctors get paid a lot of money so don't feel that you are wasting their time.
I have recently been to see my doc about panic attacks and feeling stressed out. Even though the day i went i was feeling fine!
Go to the doctors, honestly.
Good luck and take care. xx
(P.s- the fags won't help it! ; )