Sounds like your bowel is spasming. I've had this a lot of times in the past 5 years. It could a range of things like dehydration, low magnesium, mental stress, physical stress like lack of sleep and sitting directly on your rectum for too long can cause rectal spasming, too much fat, caffeine and other foods that irritate your gut (for me that includes popcorn and chocolate). I suggest going to listentoyourgut.com for tips on proper elimination positions, abdominal massage and good gut bacteria