Welcome to HCM forum.
It seems that your child could be most probably having antibiotic associated diarrhoea
however a viral gastroenteritis
needs to be ruled out.
If its due to the antibiotic omnicef
it should usually resolve on its own after stopping the omnicef within a couple of days ( max a week).
If it persists or worsens then do consult your doctor to rule out a gastroenteritis .
As of now probiotics and zinc supplements as per your doctors prescription may help in faster resolution .
Meanwhile make sure to maintain good hydration by giving her plenty of fluids .
Wishing your child a speedy recovery .