When my son (10 mo. old, CAH saltwasting) started eating more solids he became very constipated. I finally called his dr. because I was so worried and he was having so much pain. I learned quite a few things (1) there were some foods I didn't realize were constipating e.g. applesauce. I had been giving him applesauce daily, he loved it. (2) there were other foods I didn't realize could help. The dr. told me to try to remember the "p" foods, peaches, pears, prunes etc. (3) since that time I have noticed quite a few friends with kids who get constipated when they start more foods. We went heavy on the "p" foods for a while, and no bananas or applesauce or carrots. After about 2 months his system regulated. I still make sure to give him some prunes/pears each day. Sorry, I don't know anything about antibiotics.