ultrasounds are not 100% accurate.
some doctors have been sued in the past for that little baby girl ending up being a boy or that boy being a girl.
when i had my ultrasound last year, the u/s tech. was not even authorized to tell me the sex, and i had to wait until i saw my OB/GYN for her to confirm the sex.
and i got what i was told i was getting, and i had a feeling it would be what i got too, a little boy.
and even she told me it is not a 100% thing.
hopefully you get your little girl though, good luck!