Ovulation BBT question

Z
So Glo and FF says I ovulated on the 18th but I still had two low temps on the
 19th and 20th before it shot up on the 21st. I read that you ovulate on your last lowest day, which would be the 20th. Is that correct or could I have ovulated any of those three days? (Got a positive opk on 17th) 
Thanks!

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