I have diarrhea. what could be causing it?

Virus, bacteria, med: Diarrhea can be acute (short term) such as in viral or bacterial infections or it can be medication induced like overuse of antibiotics or laxatives. Usually replenishment of fluids and electrolytes and avoidance of certain meds can stop the diarrhea. Diarrhea can be chronic (long term)if associated with lactose intolerance, inflammatory bowel disease, or irritable bowel. Tests are usually needed.

More info: Food quality and quantity, new diet, allergies, intestinal infections, food poisoning, chronic GI problems, short intestine after bowel surgery, many reasons out there. If you need a specific answer to a specific issue, go see an internist and get a good history and physical, possibly submit a stool sample, and find out the exact cause