My dog is having diarrhea, what could it be?

The diarrhea is because of the change in her diet. It wasn't a gradual switch. So if it persists you need to put her on a special home diet of cooked chicken and rice or ground beef with elbows and feed 3 small servings a day for a few days and then gradually add her regular food to this mixture. I don't know the size of your dog so give her enough to satisfy her but not too much.