What would cause a swollen lymph node on the back of the head in the center of the neck? is that possible? and what causes it it's been there a while.

May need a biopsy: How do you know its a lymph node? It could just be a little benign fat tumor called a lipoma. Your doctor should palpate it a decide whether it needs to be biopsied.

?lymph node: That is a funny position for a lymph node. Maybe it is something different like a cyst. Review it with your doctor and obtain an US and perhaps a biopsy. All the best.

Anything: first it might not be a lymph nodes it could be a soft tissue or a skin mass like lipoma or sebaceous cyst. In case it is a lymph nodes it could be a chronic granulomatous infection rarely neoplasm. See your doctor for proper evaluation and treatment

Not a lymph node: There are not lymphs in the area you describe. A cyst is more likely. Have it evaluated if you are concerned.

Cyst: I can't say without an exam but in that location a sebaceous cyst is the most likely. At any rate, it should be looked at.