I have recently changed my diet and I stop eating beef of pork, I still eat chicken and fish. However, since I have changed my diet I have these periods late at night when my stomach starts to hurt really really bad, I mean my legs will be shaking because I am in so much pain, I have a bowel movement and it has alot, alot of blood. the water will be dark red. Then I start vomiting. What may be the causes