What is the cause of runny stool? Everytime I eat within 15 minutes I have to go to the bathroom immediatly, really watery stools what causes this? Its been happening for about a year now.

Gastrocolic reflex. Sounds like irritable bowel syndrome. Usually responds to antidiarrheal medications.
There. There are many possible causes for changes in bowel habits. For your reported symptoms, 2 of the most likely explanations are a food allergy/intolerance and irritable bowel syndrome. The common food allergies/intolerances that can cause your symptoms are lactose intolerance (unable to digest dairy products), celiac disease (inability to digent gluten/wheat). With severe lactose intolerance, people usually get bloating cramping and diarrhea. Another common cause is irritable bowel syndrome. This is a dysfunction of the digestive system where it become uncoordinated. Some people get diarrhea, others get constipation. There are many other causes from parasite or bacterial infections to inflammatory bowel diseases. I would suggest seeing your doctor to consider testing or further evaluation. In the mean time, you can start making a diet/activity log to chronicle what you are doing and what you are eating immediately before these episodes happen to see if you can find a common trigger.