As long as there's no drainage or goobers, it could be any number of harmless things. Animals are much lower to the ground than we are -- and cats moreso than dogs. It could be that there's a bit of dander, dust or pollen around that your kitty is picking up and sneezing out. A good vacuum/sweep/air purifier should help with that.

Another possibility is hair in the sinus. Perhaps while cleaning herself she sniffed up a hair that just won't come out. Or maybe she's got a tiny little hairball that causes her enough discomfort to want to sneeze. And sometimes kitties just have a sneezy day! Just watch for any signs of drainage or goop. Otherwise, I wouldn't worry too much.