White patches on roof of mouth

For several months now I've been getting these small clusters of white spots showing up on the roof of my mouth, and also on the back bottom near my teeth. These spots are painless and feel rough when I run my tounge over them. They come and go, as in I've got two patches on the roof of my mouth but I'm sure by tomorrow either they'll be gone or they'll move somewhere else. Any clue as to what this could be? I plan on visiting my dentist this month (he's out on vacation) but before then I'd like get some kind of bearing as to what this could be.