Hello, i read your question carefully. Metronidazol (flagyl) is an antibiotic. Antibiotics are drugs that are used to fight bacteria. The gastrointestinal tract holds bacteria that live symbiotically (give and receive) with the human body. When antibiotics are used to fight a kind of bacteria that caused some infection in the body, even the bacteria of the normal flora can suffer the effects and the organ or system can be troubled. In the case of metronidazole, it has greater effects on the stomach and guts flora and that s why you feel the pain in the belly, and diarrhea or constipation can happen. You don t need to be worried, in some days everything is going to turn to normal. I hope this answer was of help to you.