What Causes Delay In Periods With Negative Ultrasound And Positive Urine Test?

Hi my last LMP was 14th Dec 2010. And I had a urine test on 30th Jan and it was negative. I again test on 7th Feb and this time it is positive. But today (9th week) my u/s says no sac found. Am I pregnent or not? If no then why my periods not coming? And if Yes then why Sac not seen in U/S report.