What Causes Headache?

I am a 25 year old female. I'm probably being paranoid, but I just wanted to check on a few symptoms I've been having. I've had headaches pretty bad for at least the past year. They are sometimes severe and usually in the back of my head. I don't have them every day, but when I do I sometimes get nauseous. Today I noticed my left hand began shaking uncontrollably. It didn't last all day, but it was bad enough that I had trouble holding a piece of paper. Could this be serious?