Dip in BBT but negative ovulation test?

I recently came off of birth control last pill taken was the 13th and next day I started tracking my BBT then the 19th I had a really low dip so I took a ovulation test it came back negative and negative for the next two day after. What is this dip is it just a fluke or did I miss my testing window? I’m new to this BBT and just trying to learn. Thank you!!!