I'm 4 days late on what is usually a clockwork 27 day cycle

I had a bfn this morning not even the faintest line so I guess that I'm not oregnant

I could have had a late ovulation but that's never happened before, I suppose it would be typical given we are ttc for things to play up.

Isn't it all such a guessing game. I have booked into the Dr for a few days time just in case something is not right.

Any ideas? Reassurance?