I think you're normal although I think there must be some people who just go through life blocking out all thoughts of it.
I had an ectopic pregnancy 2 weeks ago and, while it was stressful and sad, I thought I was fine with it and grateful to have got through it relatively unscathed.
Then about a week ago I had the hugest panic attack about just the things you describe. It was a panic attack but, like you, the thoughts wouldn't go away straight away and I got v upset about it for a few days afterwards.
So I guess I'm saying a few things - firstly that you're not totally abnormal. Secondly, I think the thoughts are linked to an event, even if it feels unrelated. Thirdly, the thoughts can fade and you can go back to living your life without them bothering you all the time. However, if they persist, it's definitely worth speaking to a GP about it as it may be some sort of anxiety that you could get help with.
It's always a difficult area, because it is an issue that it's actually quite rational to worry about and nobody can really come along and 'fix' the problem. But it does nobody any good to fixate on it on a day to day basis, so do seek help if it is not improving.