Am I pregnant?

So a few weeks ago my SO and I had sex, and he finished inside me. I'm on the pill and then was kinda freaked out about it and also took plan b the next day. But this week I've noticed a light brown discharge. Everything I've read says its implantation bleeding but I took a test this morning and it was negative. I'm supposed to get my period in 4-5 days, so I'm not 100% if the test was accurate. I have no other symptoms. Any ideas if this is pregnancy?