I have a lump in the middle of my upper trapezius. It's been there for about three years and did not case any pain or discomfort until two days ago. Now it's very tender to the touch, about the size of a large marble, firm and does not move around.

It is most likely a lymph node and if swollen it is a sign of a minor infection in your body, like a cold, flu, etc. Enlarged Lymph nodes are the body's early warning signs of a low grade fever coming one. There are about four or five located in different parts of our bodies.

En.wikipedia.org/wiki/Lymph_node