sandk,
Unfortunately, it is not really possible to give you too much information, since you gave so little details.
Where in your are is the pain located, is it in the shoulder, or somewhere in the upper arm, the forearm, etc? Is it constant, or only when you move the extremity? Where is the lump located? Is it painful? If so, is it constant, or only with activity? Do certain movements make the discomfort worse? What type of pain is it; sharp, burning, electrical, dull, aching, etc?
If the problem is getting worse, you may want to have your physician take a look at it. These are all things that he/she will ask. With a thorough evaluation, then the cause of your problem may be determined. It is not really possible to make a diagnosis over the internet.
Good luck. Hope you find out what is causing your problem.