I have a pea sized hard nodule to the back left side of my head. It is not visible as it is covered by my hair, it is not painful and does not appear to be growing in size. I have had it for about 8 months or more. What could this possibly be, there are no headaches associated it with it.

Depending on the exact location it could be a swollen lymph node or possibly a sebaceous cyst. To get a definitive diagnosis you should have it examined by your physician.