I know this will sound weird, but – is it possible it’s gas? Sometimes I get pain and tightness in my chest, and belching does much to alleviate it.
Isn’t that pleurisy? If the pain is acute/sharp see a doctor asap, but if you just describe it as discomfort that may not be necessary (could just be that your muscles aren’t used to taking full breaths). During an annual visit, with a stethoscope, the doctor will ask you to take a deep breath and listens for the sounds of pleural lining from your lungs grinding against eachother (among other things).
~Max