Brief Answer:
I'll mention some potential causes

Detailed Answer:
Hello,

you haven't given many details about your situation. Some important aspects about your problem may include the following:
- having diarrhea at night (awakening you)
- having other symptoms (like pain or fever)
- relation of diarrhea with the ingested food or drinks (eg chewing gums, drinking refreshments etc)
- diarrhea free days
- weight loss
- previous surgical operations
- medical history besides the diarrheas (drug use etc)

I'll mention some causes of chronic diarrhea and suggest my opinion about the best action for the investigation.

- infectious causes (mostly parasites) : stool tests for parasites would help with the diagnosis
- inflammatory bowel disease (Crohn's disease and ulcerative colitis) : colonoscopy and biopsy are needed. Blood tests may help indirectly.
- irritable bowel syndrome : usually related to psychological stress, it won't awake the patient at night. This is a diagnosis reached by excluding other diagnoses.
- colon cancer : conoloscopy and biopsy for the diagnosis.
- microscopic colitis : colonoscopy+biopsy needed for the diagnosis.
- Celiac disease : gastroscopy + biopsy and antibodies can be used to reach the diagnosis.
- lactose intolerance : diagnosed by excluding other conditions and by noticing that the symptoms appear after ingesting lactose.

The list could go on...
So after reading the above, I guess you'll be aware that the regardless of the cause, examining the stool for parasites and endoscopic evaluation of the intestine (and perhaps also the stomach and duodenum) is essential for the diagnosis. General blood tests can provide clues to the diagnosis and assess your general condition.

I hope I've helped!
Contact me again, if you'd like more information.

Kind Regards!

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Brief Answer:
Since you're already diagnosed, I can't add anything more...

Detailed Answer:
Thank you for providing useful information! Let me comment on the data you've presented.

Shigella has nothing to do with your current diarrhea problem.
The same applies (usually) for polyps. There are certain cases that diarrhea may be the presenting symptom though.
Irritable bowel syndrome is definitely one of the most common conditions and a very frequent cause of diarrhea.
Lactose intolerance gets worse as the years go by but it's effect is limited to diary product consumption.
Allergies may also cause diarrhea, so avoiding the already known allergens is the only solution.
I don't know exactly what you mean by "colitis" because this is a vague term. Most people mean the irritable bowel...

So in conclusion, you've got more than one reason to justify your diarrhea. The best you can do is to try to find the food pattern that won't irritate your bowels (increased fiber or low fiber, etc) and avoid food that you know it will hurt you (like wheat or milk).

I understand that you must be frustrated with this situation but I think this is the best thing you can do. If your tests haven't shown anything else, then you've got to focus on the already diagnosed diseases.

Kind Regards!

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