Since it has been quite a while and the lump has not resolved, I would consider getting a CT scan to rule out a skull fracture, or bony cyst due to the trauma. It's hard to tell without examining you, or knowing how hard you hit your head, but a CT should tell you if it anything significant. You can see your primary care doctor to arrange the test, and then he/she can refer you to someone else if necessary.
Good luck.
JMK MD