I have a very sensitive stomach too. I was on Tramadol for 2 years, but during that time I suffered very bad constipation, then once every few weeks, or more if I ate spicy or rich foods, I'd have the most horrendous diarrhoea [spelling?] - it would invariably leave me collapsed on the bathroom floor. I also lost my appetite for those two years and was nauseous 90% of the time. I only kept on taking it because it was the only thing that gave even the slightest pain relief.
I hope it works out for you though - good luck!