12 days late

I am 12 days late and keep testing negative, I am hooked in to see my doctor next week so I will ask for a blood test but has anyone else had this problem before? I have a lot of pregnancy symtoms and don't feel like I'm going to come on my period so I am convinced I am pregnant but still negative on the tests! And 12 days late on my period seems like I should be showing positive by now - can anyone advise? X