3 urine tests negative

Ok so first things first - My husband and I live in different countries currently and I gave birth on July 17, 2013. The I got my first period afterwards on September 6,2013. I went to visit my husband on September 23,2013 and had sex on september 24, 25. On September 25,2013 we were using only a condom and it broke there was no ejaculation but within 48 hours I took postinor 1 which is Plan B in that country. Then I had sex with condom on september 29,30. September 30, 2013 is the last day we had sex. On October 3, 2013 I got my period after taking postinor and then later on i Had pains so I went to the doctor and did a pregnancy test with urine and was negative this was october 22,2013. On october 28, 2013 I got a very light period but stil had minimal cramps and then did an Ultrasound on November21, 2013 transvsginal and abdominal plus 3 urine tests negative, the doctor claims that there was nothng indicating pregnancy on the ultrasound but im sooo scared because I didnt get it yet since oct 28. Is there a way that the US didnt detect anything? Is it because of the postinor? What should I do?Would these things detect pregnancy?