Negative pregnancy blood test but still no period

I am never late on my periods. I am on a 23, 24 days cycle. I had unprotected sex three weeks ago. I am now four days late. Went and got a pregnancy blood test and it came out negative. If I'm not pregnant, then why is going on?
The doc says for me to go back in two weeks if I still hadn't had my period. I am a person with very low patience, this is driving me crazy.

Negative pregnancy blood test but still no period