There are many things that it could be, and I really think that you should go and see a doctor, even if it means going to the e.r. Because no matter what your age, it could be a stroke or even a heart attack. It could be a swollen lymph node, or a torn ligament, anything like that, even having a shoulder out of joint. Please go to the doc. Hope this helps, good luck to you.
This could be any number of problems. Do you have a fever? Is there a rash or can you feel a mass? It could be a pulled muscle or it could be some more serious such as a problem with an axillary lymph node. It may also be indigestion. If it does not go away within a few days, you should consult your family doctor.