Why do antibiotics make me constipated?

Altered flora. Diarrhea is much more common with antibiotics than constipation. However, any antibiotic can affect the good bacteria in the large intestine thus affecting gastrointestinal transit.
Flora and fauna. Antibiotics kill good bacteria as well as bad. The gut requires "normal" bacteria to function properly. When they are reduced with antibiotic use, problems can occur. I always suggest the use of probiotics whenever antibiotics are taken to try to prevent this from happening.