A food parasite would be found in the stool samples. Immodium is anti-diarrhea medication.
Colestid is mainly used to reduce the cholesterol level in the blood but can also be used for treating diarrhea caused by gallbladder removal. Colestid should not be taken in cases of rectal bleeding!
All medical investigations (blood and stool analyses, CT-scan and colonoscopy) excluded an organic reason for the symptoms you are experiencing (infections and tumors). The only possibility left seems to be irritable bowel syndrome (IBS). IBS is a functional disorder that manifests with: constipation, diarrhea, abdominal pain, bloating, gas, belching, heartburn, reflux, difficulty swallowing, an early feeling of fullness with eating, and nausea. Treatment of IBS is very complex and includes: Monitoring (identifying the factors that provoke the symptoms); Dietary modification (avoiding some types of food and increasing dietary fiber); Psychosocial therapies (stress-treatment); Drugs (anti-cholinergic drugs, antidepressants, anti-diarrhea drugs, anxiolitic drugs).
You can consult an internist about your condition, seek out treatment alternatives, and follow up on your condition.