Jodsan, . I have tried googling the same thing and from what I found the cervix can actually be a different positions at different times of the day so they recommend if you use CP to predict O or whatever to check at the same time of day.

I used first response last time I fell pg at 12 dpo in the middle of the afternoon and got BFP. Everyone's bodies are different though but I have had success not using first morn urine. Maybe wait until Tue or Wed to test again. It still could be too early - you never know. I won't say sorry about BFN yet because until af arrives.....