3 weeks late....

I usually have a very normal cycle, every 24 days. My LMP was 5-24-07. I was supposed to comeon 6-17. I have since taken a blood test a week later, which was negative. I then waited another week (last week) and the Dr. tested me again and it was still negative. Everyone says that its probly just stress, but have you heard of any other possible outcomes. I go back to the Dr. in the morning to have an ultrasound. I kind of scared because I have NEVER missed a period before without being pregnant. I have alwasy had regular cycles. Any thoughts? Could it be early menapause because I just turned 30.