2 weeks late; multiple negative HPT's

Why is this happening?! I understand it could very well be stress, but I'm not any more or less stressed than the average American, so what's the deal here? I also read that I could still be pregnant (my bf & I are thinking of conceiving) & I definitely have pregnancy symptoms that could definitely be confused with PMS. Ugh! I have a Dr's appt next week & I'm going to ask for a blood test, but talk about anxiety! I'm never late! Has this happened to anyone else? What was the outcome?