what is up with my dog?

  every time I rub her belly she will sneeze
This is a funny question because I know just what you mean, mine do it too. It is a natural thing for them to do. Think of it like this, when they are on their back, what moisture there is in their nose drips back into the sinus cavity entrance, causing them to sneeze it out as if they had inhaled dust or something. It is sort of like a reflex reaction. Nothing to worry about, just know its coming and move your head to the side. LOL
Most likely, and it does sound funny, she's smiling. She's probably happy that you're rubbing her tummy and is trying to smile. Does she show her teeth? Almost as if she's trying to growl? My previous dog used to get excited every time we would come home and smile which would send him on sneezing fits. It's really cute.=] Nothing to worry about or anything though. Hope this helps some.
Probably she is allergic to something you were or allergic to you. I not kidding my dog has the allergic test and he is allergic to human dandruff.
click and reward. teach her to sneeze.