Hi WOL,
What you are probably describing is the "mastoid process", a bony protuberance in which many of the neck muscles attach to. Another possible cause is a swollen lymp node, part of your body's immune system. Either way though, if you have no symptoms present such as fever, there is probably relatively little if anything to worry about. Though if it continues to bother you, or it becomes at all tender to touch, talk with a doctor or have one look at it.
Take care.
