PTSD, Borderline Personality, and (especially) Bipolar Disorder all have night terrors as a common side effect. I'd say tac it up to night terrors and stop taking Ambien. The Ambien will actually make your nightmares worse, because of what disorders you suffer from. It can also cause things like sleep-walking/cooking/etc. as well as enhanced psychotic symptoms (read "hypo-manic").