period or pregnant?

so i’ve been on birth control for about 3 months and have been sexually active for about the past 2 months. i got my period both times. today, my period should be coming. for the past week or so i’ve gotten brownish discharge and i had cramping yesterday. now it’s reddish pink but not a lot at. i am used to having heavy periods, but sense taking birth control my period has been very light both times. the only way that i could be pregnant is through pre-ejaculate so the chances are low right? what’s going on?