Due date question! please answer?

I went to the ob/gyn today and she did ultrasound to see how far i am and make sure nothings wrong well last thursday took home preggo test and friday went to a pregnancy resource and the said 5 weeks 2 days and due oct 31st. well today the ultrasound said 6weeks and 3 days and the baby measured at 6.1mm. and that i was due oct 25th well i added from tuesday will be my 7th week to the 40th week well that adds up to be oct 26th i was wondering what due date i should go by and if u think he/she is measureing to small or to big right now? well thanks for any answers i appreaciate it abd by the way this is my 2nd child.