There are many reasons that you may only be five weeks along.
The most likely reason is your time of ovulation. You probably ovulated later than 14 days and so aren't as pregnant as you thought.
Your 20 week sonogram (ultrasound) will give a more definitive time for you.
Your doctor will measure your uterus (if you haven't seen him yet) and that's a pretty good judge, too, of how far along you are. Mine have always been right on.