I have a lump on the back of my neck. dr thinks its a node and prescribed an antibiotic but its still there and very painful. what could it be?

Soft tissue mass: It is appropriate to give a trail of antibiotic for an inflamed lymph node, although the source of the inflammation ought to be determined. If, however the mass persists for a week or two after antibiotic therapy, then a biopsy may be in order.

Lymph node: Sounds like a swollen lymph node. The fact that its painful means that it is unlikely to be cancerous. Chances are you have a mild infection. Are there any sores on your ears or scalp? I would continue the antibiotic. You should have resolution in a week or so.