Gender determination at 14 weeks

I went to one of those places that specializes in gender determination and 3D imaging at 14 weeks and was told I was having a little boy! I am now 21 weeks and my OB doesn't determine gender until 24 weeks. My boyfriend and I are both ecstatic to be having a little boy but the last time we went to the doctor my midwife said next time I come in she'll "check to make sure it's still a boy". Has anyone been told they were having a boy early on and it turned out to be a girl? I could see it if it were the other way but I don't see any denying that he's a little boy!