can CM lie?

So during my fertile week i had more discharge than usual, then near my AF due date i became dry. I had unprotected sex a day before AF was due & it's a week ago & still no period. I did a pregnancy test yesterday & it came back negative. Am i not pregnant or was there even a big chance on becoming pregnant a day before AF? I have a long cycle like 35 days so my ovulation date is more than a week from my AF date.