I've never gotten it before and I've been on pred quite a few times....I've heard flagyl can cause thrush as well. You may have to look into another probiotic that also has acidophilus but other strains as well that aid with IBD to avoid getting thrush in the future. Primadophilus reuteri made by natures way is a really good one for IBD and Sangsters brand called Pro Bio TX11 is awesome and priced better than the PR (in my area that is).
For now, you could try plain yogurt until you can get magic mouthwash tomorrow.