When you're "zoning out" you're dissociating and it is very common with PTSD. It happens to me when I'm very stressed or overwhelmed. A memory is something you are fully conscious of. Thinking about the incident all the time is called ruminating. A flashback is from the subconscious and comes on unexpected, usually as an image flashing into your consciousness. It can also feel like the incident is happening to you again. All the physical feelings, emotions, etc. can come up like they did at the time. Flashbacks can be triggered by anything similar to the incident, like you seeing your cat James, and can bring up strong emotions. I had a really bad flashback about when a man I was seeing tried to choke me to death years ago. My husband put his hands playfully around my dog's neck and I lost it.