What Causes Diarrhea After Taking Calpol And Zyrcold For Throat Infection?

Hello doctor, My 3yr old son had high fever with bodyache and his doc put him on clamp kids forte (3.5ml) along with calpol and zyrcold saying he has throat infection. After thr 3rd dosage of antibiotic, my son had loose motion so the doctor changed his medication to zanocin+econorm. My sons fever has gone down but has a bad cold now. I was wondering if his medication is right. Please help.