Late BFPS?

Can you girls share your experience with me where you tested the day AF was late and got a BFN & got their BFP a couple days later? 
I took one test 2 days before AF was due and the day AF was late and got BFNs, so I've been refusing to take another because I have a sonogram next week. I have been sweating more, very tired and my drinks are tasting off... My soda tastes just like carbonated water and OJ tastes spoiled. The only thing I'm liking is sweet tea! Anywho, AF was due on 2/3!
Please share your experiences!