A whole month behind.....

Alright I just need some more opinions here because nothing is making any sense...... So here's the deal my last period was on January 5th and I haven't bled since I took a positive pregnancy test on February 13th (exactly 6weeks ago) if you go by my period I should be 11 weeks along.... But the ultrasound and blood levels said I was only 5-6weeks along.... How is that possible when i tested positive exactly 6 weeks ago..... I would have had to gotten pregnant that same day? Not possible I was at work all day..... Also starting yesterday I've had bleeding ... Not heavy bleeding just obviously blood and pain in my abdomen.... I'm just lost and I don't know what to do am I having a miscarriage? Or am I not?! How on earth could I only be 5-6 weeks along?