Pregnant while on birth control?

I am now over 11 weeks pregnant &a; my last ultrasound showed baby okay, but I'm a little concerned. My ultrasound said I was 8 weeks on that day so that means I would have gotten pregnant on June 16th.. but here's the thing; I was on the birth control, Nexplanon up until June 20th I had it removed. My boyfriend left on vacation with his family on June 18th. We had unprotected sex on June 16th, 17, & 18th. Then we had sex again after he got back on July 3rd. Is it possible the ultrasound is wrong on what date I conceived? How accurate is it? Did I get pregnant on birth control or could I have gotten pregnant somehow right after it was removed two days after unprotected sex? Please help.