I Was Diagnosed With Herpes Simplex 2 A Couple Days

I was diagnosed with herpes simplex 2 A couple days ago. My doctor told me that I've had it for a year. I've got tested several times in the past for an STD, but my doctor claims this is the first culture she's done. What is a culture and don't they do one every time they test for an STD? What are the chances of a false positive?