@burcinc-- Is diarrhea the only symptom you're experiencing?
If so, I wouldn't worry about it. I get diarrhea from antibiotics all the time. It's considered a common symptom unless it's so severe that you're getting dehydrated and if you have additional symptoms like vomiting, rash, itching, difficulty breathing and so forth.
Antibiotics cause diarrhea and other gastro-intestinal side effects because it upsets the bacterial flora in the stomach and intestines. I suggest taking a probiotic supplement (or eating lots of yogurt) while on antibiotics to prevent this.
Usually diarrhea, vomiting and nausea are signs of sensitivity or intolerance to medications. An allergy will cause hives, itching and difficulty breathing.
I'm not an expert though, so if you're not sure, please call a pharmacist to double check.