What Causes The Blood Work To Be Set As High Risk Pregnancy?

hi my name is annisa and i am expecting our 4th child. i am 38 years old and i have taken nuchal tests and the nipt test . the nuchal said my chances of having a baby with down syndrome was very low but the second blood work set high risk. I'm frustrated and afraid now its suggested i take an amino but I'm hesitant. i asked can i take another sonogram or the bloodworm screener again the doctor said no to screener but yes to ultra sound for the second trimester. i am 18 weeks today and I'm confused and trusted with all of this testing and roller coaster results. my other 3 children are normal. they are 13, 11, and 2.no one in my family or my husband family has any chromosome disorders. what should i do?