victoria,
That's the nature of flash photography with those built in flashes. It can be so many things, mostly microscopic particles. For instance to name just a few, dust, mist, fog, pollen, rain, dew, knats, or any form if tiny particle. You won't even see them, but they're there. Sometimes it can be dust on the lens as well. It doesn't have to be a very large particle, and since they're constantly floating around they will appear and disappear from picture to picture. The next time you're near your window early in the morning when the sun light shines in almost horizontally, try to pay attention to all the little particles you can see in the air.