Due date confusion

Carly
Hello! I called my Dr to book my 1st appt as I conceived on the 15th June, making me 3 weeks preg this Sunday. I know this as my BF literally walked in the door from Afghanistan! (yay)
​The Dr told me I am 4 weeks and 5 days as she is using my first day of last period... But I know I can't be as my BF has only been back for 3 weeks this Sunday! 
Haha! Help?! What one do I stick with??