Hello girls! Could you please tell me?

Hello girls! Could you please tell me ?? the gynecologist ordered a glucose tolerance test (sugar curve) he wants it done from week 25 to week 28, if I do it later he said it will be too late he won't show. Can you tell me is this an obligatory procedure for all pregnant women at week 25 or is it something they found in my tests and want to check? The thing is that mom once did it, so there was suspicion of diabetes, and after drinking sweet glucose she felt very bad...nauseous...she couldn't go away for a whole day after that analysis. Now I am afraid...what if I also feel bad, and I still have to go