Blood Test for Herpes

I have a couple of questions about herpes. Does it show up right away or does it take a while before you know you have it? I had blood test for herpes two years ago with my last pregnancy and it was negative. Now I’m pregnant again and I have a positive reading from the blood test. Would that mean that I was infected with herpes sometime between the two babies? My second question is: How often are the blood test results wrong?