Hi Shadow,
Get that ALL THE TIME. (Been getting it for over 10 years).
When you get anxious, it causes your muscles to tighten and sometimes "twitch". Perfectly normal.
Your stomach, intestines, etc.. are controlled by muscles. When they tighten and twitch, you feel it in your stomach and intestines. This is a lot of the causes of IBS, which is a perfectly benign condition.
Important thing to keep in mind... KNOWING the cause, doesn't make it automatically vanish.
I have been stuck on that for years. Even when I know for an absolute fact that all I am experiencing is anxiety, I still get paralyzed by it. I find myself saying "I know this just anxiety, so now it should just go away". Sadly, it doesn't quite work that way. I am told that it is only once you truly accept the feelings for what they, they will start to go away. I am told once you stop caring about them, they won't happen anymore. I keep saying I am told b/c I can't seem to put that into practice, but I understand the concept.
Hope this helps!