Brief Answer:
different possible causes

Detailed Answer:
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Diarrhea is called chronic by some definitions if there are at least 3 loose stools daily for at least 4 weeks.

Infection would have been ruled out with the stool sample but there are other causes such as: (1) irritable bowel syndrome

(2) hormone related causes such as hyperthyroidism (high levels of thyroid hormone) or in persons with a history of diabetes and subsequent damage to the nerves in the intestines

(3)inflammatory bowel diseases- this refers to crohns disease and ulcerative colitis

(4)food intolerance eg lactose intolerance or other specific food intolerance or celiac disease

(5) side effect of medication

A food diary would determine if there are specific foods involved. Blood tests can be done to check for celiac and a breath test for lactose if this is a possibility for you.
Blood tests can also check thyroid hormone levels.

There are also certain criteria called the rome criteria that your doctor can go through with you to see if you have IBS (irritable bowel syndrome)

If none of these fit then imaging may be needed to check for crohns or ulcerative colitis

Summary: (1) different possible causes
(2) additional blood tests needed
(3)other tests may be needed depending on findings.

I hope this helps, feel free to ask any other questions

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