My girlfriend says she has loose stools (diarrhea) for almost every bowel movement. what could cause this?

Many things: It could just be that her stomach is sensitive to stress and her diet. Oftentimes, there can be low level food allergies causing symptoms such as these. The most common food allergies are dairy and gluten also peanuts and certain fruit like strawberries, and citrus can cause problems as well. Another possibility could be a chronic infection like giardia that needs treatment. Have her see her md/do.

Many possible causes: There are many possibilities. The most common might include: lactose intolerance, gluten intolerance, celiac disease, pancreatic enzyme insufficiency, bile salt deficiency, yeast overgrowth of the bowel, or parasites. Daily emotional stress may also be a cause. More severe and less likely causes may include: cancer, ulcerative colitis, or crohn's disease.

Many possibilities: There are many possible causes to diarrhea. Infections are a common cause, such as from a bacterial infection (salmonella, shigella, etc), viral infection (i.e. "stomach flu"), or a parasitic infection (giardia, tapeworms, etc.) additionally, other causes include inflammatory bowel disease (crohn's or ulcerative colitis), irritable bowel syndrome and others. Best to see your doctor for testing.

Loose stools: There are a large number of causes, some serious. She should see a doctor.