What Causes Stomach Pain,bloating And Diarrhea After Following Omni Drop Diet?

I was on a omin drop diet (omnitrition) for a month 500 calories only, included salad, asparagus, chicken, fish, low fat beef, strawberries and apple. I lost 20 pounds and started the maintenance phase of the diet and introduced some cheese to my system and became very ill with upper stomach pains, bloating and diarrhea. I have not tried cheese again and continue to have the bloating, upper stomach pain and light tan colored diarrhea for last two weeks.