My Dog Has Really Smelly Farts!?

My husband and I have a 8 year old 90lb Lab/Dobberman Mix and she has some of the smellest farts EVER. They smell like stale/rotten popcorn. I know she isn't eating the best food, she is on pedigree, we're wanted to switch her over to something better, but we aren't sure what? Anyone have any ideas on what would be a good food? It needs to be relatively inexpensive, times are hard for everyone, but it needs to be good for her. Also, her smellyness has just recently started, I know she isn't getting into anything in the house, our trashcan is covered and no food is left out and she isn't allowed people food and our cats litter box and their food is out of her reach, so it has to be from outside. Could cat or rabbit leavings in the yard be contributing to her stinky farts? Thanks for all your answers...