Fear doesn't have to have a memory attached I don't think. There's the phrase, "scared witless". Just being extremely scared can be a feeling in itself that will stick with you; that's what PTSD does.

Summer before last my husband and I were camping and sat on the same side of a picnic table that was one piece, all attached? We're both obese so it went over backwards and, to make matters worse, we were under the recreational vehicle (RV) awning so next to the RV and we both smacked our heads on the RV as we went over backwards and then were dazed and tangled in the table with no one but ourselves to get us out. I remember being frightened something might have happened to Scott and knocked him out, etc. I remember feeling intense anger because I hate the physical feeling of being out of control (you will fall and go boom whether you like it or not! :-) but I noticed this summer, camping that we are overly careful sitting down anywhere and out walking around campgrounds when I see a picnic table, doesn't matter if it's empty even, in the position ours had been, I get anxious. . . My life wasn't threatened, it was just a bit of a physical fall, a little bruising, etc.

Something truly serious such as you went through much younger would probably be exponentially larger but there wouldn't have to be much "substance" to the chaos, could just have been your childish "thoughts" on what was wrong and impressions of how things were, etc. and though you don't remember the exact thoughts anything that "reminds" you of that feeling, starts to feel that way (mention of your school) could set you off again.