ConfusedABit, this is worrisome, for sure. NIPT is somewhere between 80 to 90 per cent accurate for Downs or other chromosomal abnormalities. This is something for you to talk to your doctor about - there are risks that come with undergoing an amnio, and since the scan looks normal, you have some decisions to make.
It's certainly heartening that the 5th month scan looks good.
Best wishes.