What medications are you on? That "body zap" is a very classic side effect of Paxil, and maybe some others. I was on Paxil for several years (HORRIBLE drug!) and I remember my mother telling me about a lawsuit against it in which people complained of the side effects, including those "jolts" and I was shocked to hear this really odd symptom being described in a way that made sense. I'd had it for years and tried unsuccessfully to think of a way of explaining it.
You say you're laid back, so my guess is you are not on Paxil, but I would definitely check with your doctor about it being a side effect of one of your meds. Such a peculiar sensation! It's possible it's the result of anxiety, I've never heard of it as a symptom of any other sort of medical problem. It is interesting that it happens only when you're falling asleep, like there's some misfires in your brain in the process of slowing down. I don't believe these are symptoms of a problem requiring urgent medical care, but if they're uncomfortable or making it hard for you to sleep, calling your doctor is not a bad idea.
As for anxiety, I have that in spades. It's gotten much worse as my GI symptoms have progressed, but for me I think that feeling sick has a lot to do with it...feeling sick makes me really anxious, which just makes me feel sicker. My diagnosis of anxiety and panic attacks long preceded the advent of my severer stomach issues. I can definitely understand the constant need to defecate causing panic attacks. Panic attacks are often the result of feeling a loss of control of your body. For me, that fear of a loss of control is manifested in a fear of vomiting. Panic attacks are really awful and uncomfortable, and if they're interfering with your life, ask your doctor if he can recommend a good psychiatrist who can assess whether medication or relaxation techniques might be helpful in easing your symptoms. I'd be careful of the meds personally, they can be pretty habit-forming. I'm an advocate of talk therapy and other techniques (visualization, meditation) as a means of dealing with anxiety. Even just positive self-talk has been shown to be very effective.