18 dpo, 4 days late and still BFN HELP!!

Hey, 
As the title says I'm 4 days late, 18 dpo and getting all BFNs! I have no idea what's going on! I'm still having sypstoms I.e discharge, boob ache, sickness ( been and feeling), headaches, dizziness, cramping! 
There is a 3 week wait in the doctors and I don't think I can wait that long! .. 
please comment everyone 
And has everyone ever gone past 18dpo and got a BFP? Or shall I just wait for the AF to show her ugly face? 
Took this test this morning!, fmu and literally nothing!, I'm starting to lose the will to live! :( 
Xx