boy or girl

Had my 19 week ultrasound yesterday and we asked the sonongrapher to write down the sex of the baby in an envelope and we will take it home to reveal. When I was having the ultrasound I had my mother in law, my mother, my boyfriend and my sister in law in the room. they all are certain they saw a girl but the card in our envelope says a boy!!! which is amazing!! the sonongrapher assured us its not 100%. 
after finding out the sex my whole family seems to be so sure it's not a boy and that's it's a girl! whenever we refer to the baby as he/him they all say "or she/her". they all love the idea of having a boy but are all waiting for the next ultrasound to see if it still says a boy. 
I don't know what to think or how to handle this. do I just wait also? please help! I feel that I'm having a girl.. is it possible to kinda feel like you know what your having?