Head and neck shakes multiple times per day?

Over the past year I have been getting this weird sensation and it feels like it comes from a spot near where my head connects to my neck. My head will shake uncontrollably and kind of violently for about 5 seconds. I know that it's normal for people to get a little shiver up the spine every once in a while, but this feels a little bit different. It didn't happen much at first but it's happening about 4-10 times per day now. It doesn't hurt at all so I'm not really concerned, but I'm curious if there is a reason why this is happening. It is a little bit distracting in class but not to the point where I think I need medical help or anything like that. I'm a 16 year old girl and I don't really drink or smoke anything, I get a fairly good amount of exercise (although it's been less lately) and I eat right. I do spend a lot of time on the computer because of my job and schoolwork, so maybe that has something to do with it. Any thoughts?