What is this small ball like bump on back of head?

Probably normal: If you are referring to the knotin the midline at the back of the head, it is a part of the skull where muscles attach from the neck. If it is something else, it could be a lipoma or small fibroma, but should be seen by your doctor.

Get to Dr.: Hopefully is just that you were not familiar with the anatomy of your skull which probably has several lumps and feeling of a mass. Other possibility is lymph node enlargement which could be associated to many conditions. Other masses...Lipomas, skin cysts, hemaromas, etc get examined by dr.