Some people have diarrhoea or an upset stomach at the beginning, it's probably due to the concentration of vitamins and minerals. It takes time to adjust to VLCD - so keep drinking water, it will help most symptoms.
just like GSQ said , diarrhea is fairly common in the first week as your body gets used to all the good stuff , then a lot of people go the other way and get bunged up , one of the lovely side effects