nervousness and exitement cause a stress response. the backbone of this is glucocortisoid and adrenaline, produced in the adrenal cortex. these are complex protien hormones. when the above are released we have reactions through out the body, however the stomach feels bad because this flood of hormones is trying to preserve the body by killing bacteria in the stomach. even the beneficial bacteria. because the stress response subsides, we gradually return to our normal physical state.hope this is a little helpful.