You probably lost a little bit of blood during the procedure and are anemic. Ask your doctor and perhaps start taking an iron supplement until you get your hemoglobin built back up. On both of my total knee replacements, my hemoglobin went down even though the surgeons both said, I did not lose any blood. But apparently I did because when I had my lab work done, first TNR I was only at 8 and second TNR my hemoglobin was 10. my normal hemoglobin is around 13.