When you are thin, you can expect to show early. After each pregnancy, the way has been paved, so to speak from other pregnancies, so with me, personally speaking because I am currently pregnant with #4, I showed early with each one. Doctors calculate pregnancy from the first day of your last period...so you would count from that point to today...that will tell you how far along you are by your doctor's calculation, not the time you "think" you conceived since it's impossible to know exactly when conception occurred. Most doctors won't see you until you are at least 6 weeks anyway.
Congrats!