hi has she anything else. like running eyes or nose? if she has you need to get her to the vets asap. if not then it could just be a bit of dust. or if you use a air freshner it could be that. is she sneezes too much then she can get a nose bleed. let me know
If she goes outdoors then it could be a seed of some sort, usually grass, which has got stuck up her nose.
If we get a cat sneezing that much in our cattery, then they are whisked off to the vets pronto.