I've just found a a spot on the back of my head that whenever i happen to find it hurts like i've bruised it but it's not a bump or visible.

See an osteopathic doctor: It may be difficult to determine why this is happening unless it is physically examined by an osteopathic physician. They may find that there are certain structures on the skull that may be more tender than others that may be on a suture line, or edge of a bony prominence on the skull. You receive more information, once this examination is done.