What Causes A Swelling Between Collar Bone And Base Of The Neck?

About two and half months ago I twisted my left shoulder while trying to retrieve a box from a shelf. The shoulder became sore, and I would feel aching down my arm and discomfort on the left side of my head. The aching and discomfort dissapeared about a month ago. Two days ago I notice a rather large swelling between my collar bone and the base of my neck on my left side. The swelling is not painful. After reading all the threads about cancer and swelling in that area, I have become very scared and confused.