Is this an emotional flashback?
I would definitely consider that an emotional flashback. I've experience them as well.
Or of connecting to the young person whose grief it was?
Yes, to your grief.
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I'm lucky to have this happen to me only privately for the most part as well. It is like being completely lost and engulfed by it. It is all consuming and feels like someone has just died. I lose touch with the current here and now. Oddly, I don't remember feeling it when I was little but I know that's where it comes from.