So sorry that you are feeling anxious about this!
First, ultrasounds later in pregnancy are notorious for more inaccurate measurements- they are still great tools and can help pick up on things, but they also can be full of errors.
Second, all babies are different- I have had some measure small, some measure large, and some measure right on- if your doctor is not concerned, don't let a Google search worry you- everything sounds bad when you Google it!
Third, I know how scary it is to be told that you have a 'marker' for down syndrome after an ultrasound- I had been told this by a doctor for our third. Those markers are just little 'flags'- they could mean nothing, or it could mean something more. You have the option to ask your doctor for more tests (at this point, an Amnio test could be done), but if they have not picked up any other warning signs previously and you have this concern based off a Google search exclusively, it not likely that the follow up tests would be needed (amnio does carry some risks and they typically only use it in the third trimester to check babies lung development).
Talk to your doctor about your concerns and let us know how you are doing!