I have a tender spot on the back of my head. what could be the cause?

Probably nothing: You have probably bumped or irritated the "spot" in some manner. Have someone look to see if there is anything visable. If not, give it a while. Remember, in medicine, something bad usually does not go away and gets worse and becomes apparent, if it goes away, it was nothing to start with.

Get it checked: Several causes are possible including trauma, a plugged hair follicle or the beginnings of a cyst forming. Other causes are possible as well, some of which require medical attention. If the tenderness persists, and/or a lesion develops in the area, best to have it checked by a physician.