What causes constant diarrhea?

It could be any number of things:

Viral infection

Bacterial infection

Parasites

Bacterial overgrowth

Food allergy

Lactose intolerance

Crohn's disease

Celiac disease

Ulcerative colitis

Change in diet

If it doesn't clear up soon, see a gastroenterologist.

I agree w/ Dr. G that yogurt might be helpful - just one cup a day - no fruit in it & make sure it has the live culture. Here's a diet that will stop the diarrhea: white rice with garlic. If you like garlic, dice up a clove for every 4 cups rice. If you have a hard time with it, put it in before you cook the rice. Use salt to help you retain some water. Your joints hurt from dehydration. Take a 99mg tablet of Potassium with each meal. Add liquid trace minerals to your water. Drink 4 ounces of orange juice. Use blackberry leaf tea for diarrhea. As to what would cause it, I suspect a mild case of food poisoning. Can also be caused by contaminated tap water, so drink only bottled water for a time. Gum or mints sweetened with xylitol can cause this if overused. Also some people have this reaction to "olean" substitute for fat in some fat-free products, especially potato chips

diarrhea is most often caused by a loss of the normal bacteria in your intestines, they help the food to digest, without them, you have diarrhea, the food passes thru too quickly. Eating yogurt will help to replace the normal healthy bacteria you need to digest your food.

could be infection, food poisoning, parasites, food allergy, reaction to drugs, nervousness, mixed food - a dozen or more possible diagnoses. best go see a doctor, have the stool analysed, make a proper medical exam, tell him all your medical history.

Bad diet or sickness.