confused about due date.

hey im just wondering about my due date and if this has ever happened to anyone else. At first my due date was march 19th, then after a few doctors appointments it was changed to march 12th. All of this added up according to this app. My last appointment on January 12 I was 31 weeks and 4 days according to this app. I got measured and doctor said i was also 31 weeks and 4 days but my due date was march 1st now. when i put that into this app, it put me ahead a few weeks. According to the March 1st due date, I am 35 weeks and 2 days. Is that correct or did my doctor measure me wrong? Going to ask at my next appointment. Thanks :)