Can My Doctors be wrong about my due date ??

My LMP was Sept 22/06 and the my due date he gave me 1st was June 29 /07 then he gave me July 16/07 which i think is wrong .. i feel like the baby is droping now and it's only May 9th .... I feel kinda abd i dont know what is going on with my baby whwat should i do ? Should i ask him to do another ultrasound ??? help