Why do i have like a knot on the back of my head by my neck. its no pain just seems like a piece of fat?

Knot on neck: As you state that there is no pain, likely you have a cyst, or lipoma. These do not have to be removed unless they become a worry to you or get large enough that you wish them to be removed.

Possible Lipoma: Without actually examining you i can't be sure, but it sounds like you have a lipoma, a benign collection of fat cells. Get it checked by your dr, and it can be removed if it is causing problems or getting larger.