measuring ahead?

When i first went to the doctors i was measuring 7 weeks 3 days, 4 days ahead of what I thought i was. So i changed that in my pregnancy app, but when i went again on friday she said i was 12 weeks on the dot, again measuring more than what I thought. Has this happened to anyone esle? Now i'm scared my due date is off. Another thing is i'm going to a special place for 4D ultrasounds thats not my doctors and i have to be 15 weeks, it originally was the 12th but i want to move it up to the 7th where i'll be 15 weeks 2 days if my doctor is correct, but i'm scared they wont be able to see if the gender if I'm actually more early than my doctor thought.