Newly diagnosed - millions of questions

I was recently diagnosed after my first outbreak following anal sex. I have been in a monogamous relationship for 6 months. My boyfriend and I have both been tested recently without any positive results. However, I am now learning that HSV isnt part of the normal STD screen. Is it possible that he gave it to me or that I already had it? I highly doubt that he cheated and I am trying to avoid jumping to conclusions. Honestly, Im just really confused and afraid. Help me!