Stomach problems have been the story of my life for the past year, and I believe it's all because of anxiety (specifically, post traumatic stress disorder). I agree with littlesheep however, you should rule out any gastro problems before blaming it on anxiety.
As far as natural remedies go, I have heard that ginger tea helps with stomach upset, although I haven't tried it myself yet. Chamomile and passion flower are both herbs that naturally relax you, therefore may help decrease your stomach upset.
I know what you're going through, and for lack of a better word, it SUCKS!!! Take care of yourself, and hope you find some relief soon.