What Causes A Delayed Period With Negative Pregnancy Test?

im 15 days late for my period i took a test and it was negative. then took another one in the morning and was possitive. then went to the doctors 2 weeks ago and took another 1 and it was negative. they did a ultrasound on my and saw somthing but didnt no is i was pregnant or if my period was coming. so am i pregnant? its been 2 weeks since then and still no period.