Zoning out can be part of anxiety, and anxiety is one part of PTSD. As far as I know, it is not in itself a symptom.

It can also be part of depression and ADHD. Or even just being spread too thin and stressed. So there are lots of things that can cause it.

A flash back is a memory that is so vivid that it feels as if the trauma is happening again. It can also be experiencing physical sensations that bring you back to the trauma. For example, as a survivor of rape I once experienced a very real flashback as a result of being held down. I couldn't see the memory in my mind like I have with some flashbacks, but my physical response of panic was just as if I was experiencing the original trauma.

Some people will even have phantom smells when a flashback happens.

A memory that results in psychic pain is not always PTSD. It is definitely something that someone with PTSD may experience, but it is not by itself an indicator.

If you think PTSD, or even simply trauma that causes you pain, you should see someone about it. Getting a proper diagnosis is the only way to really know what will best help you feel better.