I just found out
〜たとたん, meaning "as soon as"; I already knew
なり with (apparently) the same meaning:
As soon as I lay down, I almost immedialy fell asleep
As soon as I asked "Where were you, Miki? I was worried", Miki started crying
I tried figuring out what's different, if anything, between these forms, but I can't find
なり in my grammars and the only thing comparing the two I found is this page, which says they are similar, it doesn't explain why, beside saying one asks for dictionary form and the other for short past.
Edit: Also, not sure about the differences between