Many of us go through our lives and experience many difficult events.
Whilst they may not meet the criteria for PTSD, if we experienced them as difficult, traumatic or overwhelming then it is likely that our brain experienced it as a trauma and therefore processed that experience in a different way to normal every day experiences.
You push them into a box and leave them there with the hope that you will never need to deal with them.
What we often find is that there is a trigger, a transitional period in your life such as becoming a parent, changing jobs, going through the menopause, your children being the same age that you were when something bad happened.
This trigger can remind you of things that happened in the past that are in the box. You might find that these things then keep coming into your mind.