negative test but late period. help!?

Hi all
I've never posted in here before but just after some advice if anyone has experienced this before at all. 
I did a pregnancy test last Sat which was 12 dpo and it was bfn :(
Af was due Monday 23rd and as yet she hasn't arrived. I'm going to test again in the morning I think. I'm trying not to get my hopes up though. X