National origin discrimination is illegal, which means that they can't refuse to employ you, or take other action against you, because of where you came from, so long as you have legal authorization to work in this country. An employer may, however, question an employee about factors relating to her eligibilty to work here, because they have a legal obligation or duty to only hire those who can work here legally, and in particular, they may do so if there is some alert on or about your immigration status--even if you believe it is wrong or incorrect, they can, and should, question you about it to make sure they are complying with their legal obligations. If there was a red alert as you write, therefore, their questions appear to have been legal and appropriate.