In the context of social anxiety disorder (SAD), post mortem refers to the period of time following a social or performance situation in which the sufferer selectively remembers only the negative aspects of the event. Typically the person with SAD will relive those negative aspects of the event over and over in his memory, strengthening feelings of low self-esteem and inadequacy.
Examples: Immediately after giving a speech and for days after, Tim could not forget how his hands shook, his heart was racing and he was short of breath. He was sure that everyone could see how nervous and anxious he was during the speech and that they wondered what was wrong with him. They probably also noticed how he forgot what he was going to say halfway through and stumbled over his words. Everything that he remembered about the speech confirmed how badly he thought it had gone.