What Causes Diarrhea And Nausea While On Antibiotics?

I have a question about a case study I am currently working on. A patient was recently treated with antibiotics for a severe bacterial respiratory infection and has come in complaining of watery diarrhea (more than 10 times a day), abdominal cramping and pain, rapid heart rate, nausea, dehydration and general discomfort. His respiratory infection is gone and lungs are clear, instead the patients colon is inflamed. The patient says the symptoms began after two weeks on the antibiotics. I am asked for the etiological agent, which I believe to be the antibiotics. I am also asked for the potential cures for the illness and how it can be avoided. I believe to be over-the-counter medications such as anti-itch lotions or anti-diarrheal medications, but not sure about the avoidance other than trial and error.