Am I pregnant?

I had sex a week ago and my period was supposed to come on a couple days ago but it’s “on” now but it’s just brown discharge like the old blood from my previous period. We didn’t use a condom nor am I on birth control. We used the pull out method. Horrible I know. It’s been two days where I’ve had brown discharge and that never happens for me. If it isn’t pregnancy could it be a STI? I’ve only had sex with one guy and he’s only been having sex with me so just help please!!