Hi there,
How long ago was your surgery? You could get anemia after major surgery and it can cause dizziness/hypotention/orthostatic hypotention.
Eat Liver/Kidneys for a few days and eat salty nuts or if you can get your hands on marmite/vegemite. If you feel better after you've had some foods rich in iron, then you have a diagnoses!
And I agree with Flycaster that if you are taking beta blockers, that hypotension is one of the side-effects.
After open heart surgery, you were probably instructed to take a beta blocker drug. They really help control blood pressure, but certainly can make you dizzy when standing up after laying prone. There are other drugs that do the same thing. I'd certainly suggest you notify your doctor at the earliest opportunity with your symptoms.