L.S.
I used to get sick at night, too. Then I realized it was my vitamins that was causing it. If you take your vitamins before bed, you should switch to taking them first thing in the morning. They're best absorbed on an empty stomach, so maybe you could take them between lunch and dinner instead?
If you're not taking vitamins before bed, it could very well just be your hormones. Every so often I get incredibly nauseous right around when I get my period, just from hormone fluctuations.