Please ignore my first comment. So from your information I see you are living in the US. Zelnorm was removed from the marked in the US because of dangerous side effects. It can cause severe diarrhea and ischemia of the bowel. You need to see your doctor immediately. This could be very serious and what you describe was why it was removed from the market.
Welcome to the gastroenterology community! I assume you have a history of constipation if you have taken zelnorm before. What is your GI history? I would recommend eating a BRAT diet of bananas, rice, applesauce, and toast and making sure to stay hydrated until this passes.