R., this very well could be a yeast infection. You know antibiotics can cause them for women, but it can in little girls too. Did your daughter's doctor examine her and make that diagnosis, or did he just tell you over the phone that it was a possibility? If he examined her, he should have provided a medicine that would clear this up in just a couple of days. If not, take her in as soon as possible! If the doc can't see her for a few days, I suggest the emergency room. I just had a yeast infection myself, and I know how terribly painful and miserable it can be. I tried to treat it myself for over a week, but it just kept getting worse to the point that I actually started bleeding from being so raw. I finally saw my doctor, she gave me a prescription for a pill that I took that afternoon, and by the next day it was almost gone. One more pill did the trick for good. I know your baby can't take pills, but my point is that the doctor can prescribe medications that are much more effictive and work much more quickly than OTC meds. The sooner you get this treated, the better for your daughter. She's probably in terrible discomfort right now.
p.s. - I know you are concerned that this may be an allergic reaction rather than a yeast infection because of the diarhea, but they could be unrelated. The diarhea may be a side effect of the Omnicef, and the other symptoms from the yeast infection, which is another side effect itself. Either way, the doctor can run a test to diagnose the yeast infection for sure.