Okay, so as it stands I have two major problems with writing. The first one, like most people, is procrastination. I already have a decent handle on solving that though, so please disregard. My real problem is number two... I try to write pretty often, a lot of times it peters off into nothing or my motivation dies before I even open up my word document, but every so often it hits me. Inspiration, ideas, coherent sentences, they're all there. Everything is golden for about twenty, thirty seconds, but then something else hits me. I never realise it at first, some how I never understand until I'm literally wiggling around in my chair, but I'm urgently overtaken by the need to take a dump. I'm not shitting you, it's almost like I can tell if what I'm about to produce is quality or not based on potential bowl movements... What's wrong with me?
Ha ha ha, I actually don't know what to say. I first thought your title was a clever one, meant to draw us in with some double entendre, but clearly it was to be taken literally! Perhaps take a pen with you and put the toilet paper to good use
But that way it's perfectly primed for agent rejection! Ready for some disdainful bottom wiping. "This is literally only good for wiping my arse."
As funny and awkward this is, I actually have a similar problem. When I finally get myself sat down and eliminate distractions, I can't get very far into writing without feeling the urge to get back up and do something else. This happens mainly when I'm writing part of my novel, as this problem isn't as prevalent when I'm writing messages for an RP or a short story. To add to the frustration, as soon as I move away from the computer, inspiration strikes and comes in full force and I have to come back quickly to type it up. Can't tell you how many times I've gone to bed immediately after putting in some writing time and had ideas rush to my head. Luckily I'm pretty capable at remembering most of it for the next day. As far as advice goes, I try to get all of it down as fast as I can before heading to the restroom/whatever else I feel the need to do. It can be difficult though, since when inspiration strikes it can steamroll into one thing after another.
Are you literally sitting down to write? Are you adopting a different body position to other activities you engage in before and after attempting to write? I hear tell (not from peer-reviewed science) that certain postures can elicit 'the urge' more than others by straightening out the colon, and I think a hunched over sitting positioning is up there (hence why we do our business the way we do... unless that's just me and I'm weird... but cats and dogs do similarly... ). Would you get the same problem if you stood or lay down to write? Do you tend to sit hunched over? Could you try sitting with a straighter back (I believe that's more healthy for your back anyway)? It's also possible that you're shitting bricks because inspiration has finally struck.
If laptops could tell their own stories eh. * reminds self to flush caches * <actually thinking about that before moving on; a thread I read this morning (and wanting to sound knowledgeable on by reply) had me looking up kidnapping and somehow I ended up reading the mantra of the Una Bomber. That'd be a motive now should someone go missing nearby (or explode). I'm gonna go and double-flush. It's quite natural @Chained . Nervousness or excitement invokes the need to poop. Turd it to your advantage—I motion that you should let being in the zone take you to the other zone (with lappy so you can write as you go), yes, let it be the gauge for how inspired you are. No need to go? Then call off your writing for the day and come talk to us here.
I don't know how emotional you get when you have a writing idea - be it excitement, anxiety, or something else - but I know, for myself, moments of intense upset or anxiety are sometimes accompanied by a sudden need for the facilities. OTOH, there's nothing stopping you from putting your time on the throne to good use, reflecting and developing your ideas.
As @EdFromNY says, adrenaline gets my bowels on the move. I believe the evolutionary reasoning is that we immediately evacuate our bowels to become lighter for a more effective flight response.
Fulmars defend their nests from intruders by spitting out a foul-smelling oil. Perhaps the evacuation of bowels is in the same vein?