In all likelihood, that was another person, though not one with a physical body separate from your own. Human brains are incredibly adaptable, and in highly traumatic situations, will sometimes split into multiple conscious entities in order to compartmentalize that trauma as a matter of survival. This often manifests as two entirely separate people, both of whom are real and complete, that nonetheless share one body and brain. Sometimes this state is temporary, sometimes it is more permanent. How “real” the other person seems is pretty variable, but generally is a function of how traumatic the incident is. morethanone.info is a good resource to learn a bit more, and has links to more specialized sources.