It could just be implantation bleeding - if you are having no pain and the U/s was fine, then I wouldn't be too worried just yet. You could have implanted late, meaning that the baby will be smaller for dates than what you expected and if you aren't 100% sure of when you ovulated and therefore conceived, that will put your dates out too - especially if you have a longer cycle. Can you get your Dr to do a check of the HCG and see if the levels are rising?