You do not say where the mass is located exactly. It is not uncommon for a mass to be over the acromioclavicular (AC) joint. This is located on top of the shoulder, at the end of the collar bone.
This joint can be injured, in what is called as separated shoulder. The result looks like the end of the collar bone is sticking up.
The AC joint is also very prone to osseous hypertrophy (or bony overgrowth). This is very common in persons who do a lot of heavy lifting, and in weight lifters particuarly. This is not a cancer nor is it arthritis.
However, this joint can be affected by arthritic changes. However, most people have some discomfort in the joint. It can also be affected by a tumor, but again, these are usually painful.
A bony mass can arise from the humeral head. An osteochondroma can come off the growth plate, but these are seen in adolescents and young adults. Other tumors are quite rare in this region, and are almost always painful.
Since you are concerned about it, you should probably see your physician. The only way to know for sure, is to have an x-ray taken. Further studies, such as a CT scan or MRI, may be necessary depending upon the results of your examination and x-ray.
The acromioclavicular joint (ACJ) is one joint that is very susceptable to degenerative changes. These tend to get worse as time goes by.
Also, the ACJ can undergo osteolysis, making it become very enlarged. This is a benign process. And, is often seen in weight lifters.
If you are only having occasional discomfort in the joint, the usual management is to maintain shoulder range of motion, ice/heat as needed, avoidance of weight bearing on the hands (certain exercises, such as dips), and possible use of antiinflammatory medicine.
If the discomfort becomes troublesome, you may need to see an orthopedic surgeon. The second stage of treatment is the use of injected steroids into the joint.
If this fails, and the patient is having significant problems with the ACJ, then surgery can be done. The usual surgery is a Mumford procedure, where the distal end of the clavicle is removed. A decompression of the subacromion space can be done at the same time, if needed.