I went through that at your age. I joked with my fianceé at the time that it was all her fault that I was going grey. Well, a few months after we broke up, the grey went away. Turned out it WAS her fault after all! :D
Yours could be stress, genes, whatever. It's not something that you can fully control, as far as I know. So just go on with your daily life as though nothing changed. If it becomes an issue for you, pick up one of those Just for Men hair colourings and run that through. Don't worry, you'll be fine.
Idk maybe it is stress, maybe it is genetic. My boyfriend has been crazy stressed out this with college and I found a couple white/gray hairs throughout his hair and he isn't even 19 yet so maybe it is stress related.