Harmony test..I'm confused..

Well I was confident in the response at first when they asked I was having a boy.. Then someone told me that I should wait to confirm at my 20 week scan use it has been wrong before for women who have recently had a boy.. My son is 3 and I guess there is a chance that there is y chromosomes left from my last pregnancy... I'm so confused.. I was ready to start buying boy stuff but now I'm scared to..