What Causes Light Headedness And Fatigue Post ACL Replacement Surgery?

Wife had ACL replacement surgery two days ago. She became light headed and blackout for about 3o seconds. She has a history of getting light headed . She is taking perocet and aspirin.her last pill approximately 8 hours before incident. I took her vitals pulse 70 BP 126/74 about 10 minutes afterincident. She has not been eating much protein.