My best guess- and it's just a guess--is that you have a yeast infection, which a steroid cream might worsen. I can't think of any useful role for an oral antibiotic like sulfa. Locoid is just a weaker version of what you're already using. You ought to call your dermatologist tomorrow, since you have one, and find out the next step. If it is a yeast infection, then a simple switch to a diffferent cream should do the trick. But don't guess--get a proper diagnosis and appropriate treatment.