I need help

I'm only 18 and I had sex with this guy a week ago. I'm not sure if he came inside me but even pre cum can make you prego correct? Idk. I was actually suppose to start my period 2 days ago but didn't. I'm still don't have my period today and I'm kinda nervous because 2 days ago I checked to see if I'm on my period so I put a tampon in just being sure. I then took it out 20 mins later and seen that the tampon was browish pinkish (I know nasty) I did my research and all I see is pregnancy! Someone please help me.