Hi Margaret.
5 main causes of this post op -
1) - Anaemia, have you had your iron levels checked recently?
2) - Low B12, again have they been checked? Are you having injections?
3) - Postural Hypotension - This is a rapid drop in your blood pressure when you stand up, causing blackness, dizziness, and fainting. This can come on post op, or be worsened if it is a pre-existing condition. GP/Nurse can check your BP seated and standing to diagnose. No real treatment, just coping strategies.
4) - Dehydration, are you drinking enough?
6) - Low blood sugars, not common and tends to be accompanied by shakiness, palpitations etc as well as dizziness.
Best bet is to see your GP, and if they aren't savvy on post op issues, ask them to check for the above.