Spotting

Need help not sure if im pregnant or not.My last period was on 12/07/17 and i had sex on 12/31/17.On 01/10/18 i woke up and use the bathroom to pee n when I wiped it was pink so I thought i was goin to start my period.only for that day i spotted a little n it stop.i never been pregnant so im not sure how this works.