Obviously I'm not a doctor but try eliminating wheat, barley and rye from your diet. THis means all bread, pasta, beer, cake, donuts, ketchup, salad dressing, etc. What I am getting at is something called Celiacs Disease. Other than the leakage your symptoms could be either Celiacs Disease or Gluten Intollerance. Do a search on both issues and I think you may see what I'm speaking of. Good luck.
When you say they 'only found colitis' your actually saying that you've got inflammation going on in some portion of your large intestine. Are they treating that condition? If not, you need to speak with your doc again and find out exactly what form of colitis they found. There are several forms some of which are generally classified under the heading of inflammatory bowel disease (IBD) such as Crohn's and ulcerative colitis.