For a few days now, I've been experiencing mild nausea, almost like a slight hangover even though I only have one drink a day. Could it be my vitamin D?

It is possible that the small amount of alcohol you consume daily could be causing your nausea. It may be a good idea to temporarily stop taking your vitamin D3 supplements to see if that helps with the problem. If your nausea persists after doing so, you may want to seek further advice from a healthcare professional.