What should I do if I don’t feel well on the day of the test?
If you don’t feel well, you should not take the test. You should be healthy and prepared when you take the GMAT. It will be an ordeal to sit through a 4-hour test while you are not at your best. You may end up canceling the score anyway. So do not get hung up on the rescheduling fee (US$50 if you reschedule before 7 calendar days of the test date or you will lose the entire US$250 test fee). Your bad score might become an anchoring point later on hindering your progress and hurting your self-confidence. Meantime, a bad score gets reported to schools as all your scores in the last five years will be shown.
However, if you are sure that you can achieve a high score based on your prior practice tests, you should take the test even when you don’t feel well. That way you don’t waste the test fee and have a chance to experience the real test while having the option to cancel it at the end.