I had my 1st dating ultrasound on 5/23 dating me at 15 weeks 2 days edd 11/11! i didn't have unprotected sex until 2/26. could my ultrasound be wrong?

Ultrasound dating: That sounds about right. If you conceived on 2/26, you are about 15 weeks along. First trimester ultrasound is accurate within 5-7 days. 2nd trimester ultrasound has accuracy of +/_ 2 weeks. 14 weeks is at the beginning of 2nd trimester. Also, remember that a 15 week pregnancy is a 13 wk fetus. The dates begin on the 1st day of your last period, not the date of conception.