Pain on breathing- what could it be.?

Pleurisy. Can be caused by multiple things: pneumothorax (collapsed lung), pulmonary embolism (blood clot to lung), pneumonia, inflammation around lung, etc. Some of these can be dangerous so I advise getting it checked out asap.

Related Questions

I have severe chest pain when breathing. What could be wrong?

Chest pain. Severe chest pain on breathing in a 33y/o is usual pleural inflammation and generally will go away by itself. If it doesn't should consult your doctor to see if something else is causing it. Would usually resolve in a week or less.