I was a week late for my period and started it yesterday but not sure if it was my period because I had sex on the 18th of November and walked into the bathroom to find a puddle of fluid that I know wasn't blood and after that incident I BARELY bled at all

Would not have any signs of pregnancy from sex on the 18th, puddle of fluid after sex is not really a sign or pregnancy.

A missed period is a sign of pregnancy, if you miss your period wait 10 days to 2 weeks and do a pregnancy test