I woke up this morning and felt very dizzy and off balance when I stood up. I also feel nauseous when this happens. What can cause this?

Many possibilities. You may have middle ear infection with vestibular dysfunction, labyrinthitis, or other acute processes affecting the neurological mechanisms which tell your brain where your body is located. See your doctor and get this taken care of. Hope you feel better soon.
Orhtostatic hypotens. If u r one a blood pressure med the dose may need 2 b adjusted. 2 low of a BP will cause this symptom. If not on BP meds ur blood pressure has a natural drop when u stand up due 2 pooling of blood in the legs. It is usually when 1 gets up fast. Try sitting on the edge of the bed a short while & then stand up. 2 b on the safe side c ur pcp & be evaluated. May need a 24hr BP monitor.