If you are as heavy as you say you were, finally getting some physical activity can truly do wonders to one's body. I weighed about 250 when I first went off to school in August, and after taking up swimming at our gym, ive managed to drop about 15 pounds, as well as see a lot more muscle tone everywhere.
As for your constant shitting, exercise speeds up all of the systems in your body, including your metabolism, thus your body processes more, making you release more.
Or I could be totally wrong, and that sickness you're feeling is a tape worm o_O