Answer:
Hello my name is Dr Roberts, if I was you I'd go straight to your GP and he can the follow the correct procedures whether it would be, a ultrasound / MRI scan etc. It could be many things it could be something as simple as a bruised section in your thorax or it could be something as serious as pneumothorax! There's also endless causes that will help the the medical staff identify what the problem is, so before you go to the doctors think of straining you've done recently (gym? manual handling?...). If you also suffer from a nasty couch you should urgently get help as a severe cough can sprain the chest muscles!
Hope I've been some sort of help
Dr Roberts