Sore spots on the roof of my mouth?

A few days ago, I noticed that the roof of my mouth was sore (stinging and painful when I eat, not the achy kind of sore) behind my front teeth. I've had that feeling before after eating hard chips or really crunchy cereal, but I can't remember eating anything that could have damaged my mouth. It's been about 5 days now, and it's still sore. I don't drink or smoke, and I'm in exellent health otherwise. What causes a sore mouth and what can I do for it? I don't really want to make a trip to the doctor unless I just have to.