Sometimes your body can ovulate at a different time than normal, possibly you ovulated a week and a half early and didn't realize it. When you do the due date calculators they usually base your menstrual cycle on a 28 day cycle. But not everyone is like that, some people have 32 or 34 or any other number of week cycles, this could push it back a little bit depending on when you would have ovulated. The ultrasound and HCG level is going to be more accurate than your estimated ovulation date though. The gestational sac can also help determine how far along you are. My first ultrasound tech told me straight away that her patients are usually off by a week or two because the body can be very random. Also, remember that the sperm can live 24-48 hours inside you, that can throw off the dates as well. Go with what they tell you and you will be fine. And like pertykitty said - your dates will change with other ultrasounds so don't even sweat it.
i should be 9w1d from my lmp, but the u/s says im 8w today. i really dont think so, i think i am 8w5d lol. just going by when we had sex, cm, o pain. but who knows, maybe i o'd much later than i normally do. they didnt see anything but a black dot at my first u/s then a few weeks later a tiny little beating heart. dont worry, your dates will change again with later ultrasounds
I know it can be upsetting do you know when you concieved like a for sure date? cause if you do then maybe inform your Doctor of that too it might help a little thats what i did cause i KNOW when this baby was concieved! Good luck!