This is wierd since I had a similar sensation on acid. I'm not sure what the actual compound was, could have been LSD, DOM, etc but it made my lower gut feel very strange and even if I just took a piss I'd still feel the urge to get up and go again. A few times during that trip the feeling was very urgent, if I ignored it or chalked it up to the effects of the drug I could sit there perfectly comfortable but if I decided I was going to piss it instantly became a race to the finish. I nearly pissed my pants and all over the bathroom at least twice from this feeling.