What Causes A Large Hard Bump On The Left Side Of The Head?
I read your question carefully and I understand your concern.
Since you mention headache let me start off by saying that it is not due to that nodule. Headache is related to the brain covering layers and vascular structures. All those structures are located and limited inside the thick bony skull, they do not manifest directly visible signs in the skin over the skull, there is no relation between the headache whatever the cause and that bump. The bump does not indicate brain damage.
What could the bump be....while tough to judge without examining it myself there are several possibilities. An enlarged lymph node is one possibility, if that is the case usually due to an infection in the vicinity (something trivial like flu or a tooth issue for example) and should subside with time.
Other possibilities are benign skin cysts or tumors. Some examples are keratinous cysts or sebaceous cysts developed by overproduction of keratin from sebaceous gland, pilomatrixoma, dermoid cysts etc. There are many of them depending on the cells they originate from and content but what I suppose matters to you is whether they represent any danger. The answer is no, they usually are left as they are, unless they keep growing, are in cosmeticly disfiguring areas or get infected. In such cases they can be excised through a simple surgical procedure, usually with only local anesthesia.
So I would not panic but simply wait and see for the moment.
I remain at your disposal for other questions.