About a week ago i discovered a lump at the back of my neck, just above the hairline to the left of centre. It feels as if it is on the muscle, is fairly firm to touch and doesn't seem to be moveable. I am posting here because i didn't know what other area to check. It doesn't hurt, and my girlfriend said there appears to be a small brown spot on top of it.

Is this something i should get checked immediately?
It sounds like you could have a sebacceous cyst. These lumps can and do form at the hairline. They are stopped up oil glands and will get easily irritated by shirt collars or hat rims rubbing against them. They can get rather large and with it being near your neck and nerves in the neck, getting it checked would be a good idea. If the doctor thinks it could be something else, like a melanoma etc, then he/she will refer you to the proper doctor.