I'd call or get her to a pediatrician. Very foul smelling diarrhea/gas can be from an intestinal bug called "rotavirus". My kids have had it (a couple of times) and it was awful! It's also very very very contagious so make sure you wash your hand with hot soapy water very well after you clean her up. Make sure everybody in your household takes extra care w/handwashing as well.
With rotavirus and the diarrhea that comes with it you have to be very very careful to keep kids hydrated. It's not uncommon for kids to end up at the hospital needing iv fluids because of how quickly they can dehydrate from all the diarrhea and vomiting. Clear liquids, no milk or dairy, no apple juice. I hope she's feeling better soon!