Omnicef and adverse reaction

My seven year old son just got out of the hospital after an adverse reaction to omnicef. It started with diarhea that became nonstop until he dehidrated. I stopped the med. on Monday after a stool test was taking that came back neg. for cedif(spelling- bad bacteria overgrowth). By Friday I had him back at Dr. because of nausea and severe stomach cramps. We were told it was probably a virus. By sat. a.m. we were at the ER and they ran the stool test again. This time it came back positive. He was so sick by this time that he spent two nights in the hospital with IV's. The scary thing is the antibiotic caused this and another antibiotic was needed to treat it!