Don’t know where else to post- the “afterlife”

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So 11 years ago I lost my dad to brain cancer quite suddenly. I’ve never told my son what happened to him or even spoken about it around him, other than the fact he died.

So today my son (who’s 4 ) came up to me and said

“ I met grandad in heaven before I was born , and he had an ouchy head that’s why he’s not here anymore”

This has really freaked me out , I don’t have a religion or anything and have never believed in heaven or the afterlife but this has seriously messed with my head because how else could he of ever known that and just randomly come out with it one day

Has anyone else ever had this with their child and someone close to them that’s died?