Hi,
I hope the bleeding has stopped by now. You could be bleeding for any number of reasons, for example, from subchorionic hematoma but your baby will still be safe.
You could be measuring smaller because you ovulated or implanted later than expected.
Please be reassured if your Dr. is saying that all is well with your baby.
Good luck!
It could be that implantation took place later in your cycle. As long as the US is fine and your HCG levels are increasing, I wouldn't worry. Good luck to you!