I feel sick after vegan diet?

well fruits and vegetables have alot of acids in them, especially fruits, that could cause some stomach pain if you are new to this diet. Caffine withdrawals would probably be more like headaches but that might be it im not a doctor or food specialist. I would advise you to just try and get as wide of a varitey of foods you can, because vegetarian and vegan type diets really just limit what you can eat. Our digestive systems are designed to work on a wide variety of food. Try new things.

Your body is not used to the sudden changes made in your diet. I would recommend taking daily vitamin/supplement pills in order to provide the daily nutrients your body requires. Also the lack of caffeine can be a contributing factor; this should go away after a couple weeks.