How long until I should take a pregnancy test?

I could just be paranoid but a few weeks ago-not really sure how many- I had unprotected sex with my So. It wasn't planned just kind of happened and I completely forgot about it until yesterday when I started spotting. I'm not on birth control right now because of antibiotics. I've heard that spotting can be a sign of the egg attaching to the uterus wall and I keep thinking about whether I might be pregnant again. I didn't find out with my first until I was almost into the 2nd trimester. So my question is when would it show up as positive on a test if I am pregnant?