Do you have the pain at the bottom of your ribs at any time? If so, it could be your gallbladder.
It's best to see your Dr to find out what's going on. Remar
I would see your doctor. I am saying this not to scare you but sometimes a sharp pain can be cancer...and under the arm pit is where most start. I hope this is not the case. Have you lifted anything that you can recall? You might want to go to the chriopractor and ask them to advise just in case you pulled something, it might be a cheaper way to start trying to figure things out.