Had total knee replacements, I am not feeling good, no energy, depressed, no appetite, have lost weight?

Male, 73 years old . Recently had total knee replacements back to back. Knees are fine, but am not feeling good, no energy, depressed, no appetite, have lost weight. When standing, blood pressure falls dramatically. I have passed out several times. Heart dr. Says postulate hypotension and given rx blood pressure med to raise blood pressure.somehow I think he is missing something. I freeze, sick to my stomach, just don’t seem to follow all symptoms of his diagnosis.