I can't tell you exactly what causes it. I would try a little fruit. Maybe a little coconut oil. And I'd bring a magazine or book in with me. I'm not being sarcastic, but relaxing while you go is kind of crucial. If you have to bring your phone in because you stress over missing a call or text, go ahead and bring it in. Warm fluids can help.
I totally wouldn't advise more fiber. Fiber "laxatives" never did anything for me except make things worse.