This small knot on your shoulder may be a sebaceous cyst
with mild or moderate infection. Many a times it has a small punctum on it and after squeezing it a creamy material gets exuded. You can just forget it but if it is causing concern to visit a gen. Surgeon who will drain it with wide bore needle if the cyst is confirmed, followed by local injection of a sclerosing agent. If it is a knot other than the cyst it needs to be enucleated and submitted for pathological examination.