High Uterus?

OK so today I went to my first Ob appointment and my due date is Dec. 24th! I am also supposed to be 6 and a half weeks preggo, but the doctor said my uterus is high for 6 and a half and is measuring 8 and a half weeks..... Has anyone else been told that before and what could it be because of? I also peed prior to her checking me and have yet to have an ultrasound.