You actually do not have to have the type of trauma that you read about in most medical literature to have an AC joint separation which will cause a bump on the top of your shoulder. I just had surgery for AC joint separation among other things in April......I did it lifting and using resistance bands. Please see your doctor for an accurate medical diagnosis. Shoulder pain is the worst. Best of Luck to you!
Thanks for following up...
As you can see from this link about shoulders: http://www.webmd.com/pain-management/picture-of-the-shoulder , there are many structures to the shoulder and surrounding area that can cause pain or a "growth." I think what philnoir is referring to is the accromian process.
However, we do not know your age, if you had any recent or prior trauma to this area, any pre-existing medical conditions or medications that might predispose you to tearing or promoting fractures or growths (spurs), etc.
Therefore, you need to be evaluated by a health care provider for an accurate diagnosis, which we cannot do here.
Is it a growth or the top of the clavicle? The body is rarely symmetric.
What is your age?
It could be anything. Without an exam, no one can say what it is. I suggest that you see your doctor to have this looked at.