Doc visit. could be a lung issue, could be heart issue, could be nothing more than a swelling from being hit that you do not remember.
Definitely go see your doctor. It could be one of many things, some minor, some significant. But because it could be something serious, you need to get it checked out.
Good luck!
Hi have you fell over or done a lot of lifting you could of pulled a muscle. It really hurts i would go see the doctor they can do an x ray and find out whats going on, take care.