pain before bowel movement

I remember the first time this happened to me. I was in first grade, sitting on the floor listening to a story and all of a sudden I got really hot and said I needed to go to the nurse really bad and all I could say was "MY BUTT HURTS!!!" My mom took me to a clinic right away and she said just try pooping. Turns out thats all it was and I was relieved.

This has been happening all my life. Sometimes it's severe and sometimes it's not bad, but every now and then...I'd say between once or twice a week..before I can even tell that I have to go I get an intense pain throughout the bottom half of my body and I have to sit down and hold on to something and breath. I eat pretty normally for an 18 year old and I could definitely eat way better..but I get my daily fiber and I know it. What else could this be? It happens more in the morning..