Why would i be nauseous when putting my head down?

Vertigo: There is a known entity called benign positional vertigo which manifests itself with feeling dizzy when changing positions. Nausea and occasional vomiting can be present as well. However, based on your limited description it is hard to tell whether this is what it is or something else. I would schedule an appointment with a doc. Meanwhile, read about epley maneuver and try it and see it works.