There are two answers to this - one good and one bad.
The most probable answer is that you miscalculated and you are actually 5 weeks pregnant.
Unfortunately there is a possibility that you did fall pregnant 9 weeks ago and that the pregnancy will not progress. It is possible that either the fetus is not developing, or that the fetus has died which usually means the gestational sac will shrink and appear to be not as advanced as you are.