You are experiencing diarrhea, and because the mixture of food and digestive juices (among which are bile salts that are green at the beginning of the tract and normally broken down by gut bacteria with changing color from green to yellow and to brown as a definitive color of stool) are passing too fast and it results in green stool.
In other words, it is normal to expect the stool to be green when it has form of diarrhea.

If diarrhea lasts for more than 2 days, you may visit a physician to determine the cause.
It sounds like this is a case of motility related diarrhea.