In my experience, rashes caused by an allergy to meds show up right away, not 2-3 days after the dose is complete, but every child is different. Since you're done w/the ceftin, I would tend to start looking into things like possible food allergies or eczema (which tends to get worse in the winter when it's cold and dry). Have you tried giving her benedryl, or using a non-soap (like cetaphyl) for bathing? Make sure that you coat her well w/a good cream (we use vanicream) within 3 minutes of when she gets out of the tub. If the rash still persists even with treatment, it may be time to talk to the ped about a prescription for a topical steroid cream (although I don't know what they can give for faces). You could also ask your pharmacist about over the counter products. If the rash disappears and doesn't come back I would probably avoid ceftin in the future.
Is your daughter feeling better? Hopefully the sinus infection has improved. Those aren't fun for anyone.
Is she still taking amoxi nightly? Has your pediatrician ever mentioned giving her a pro-biotic (Culturelle) to help with replenishing the good bacteria that were killed w/the antibiotic?