ugh plz help

i’ve have my nexplanon since november 2018 and i barely get my period but every couple of months i’ll get it for a week and than i won’t have it again for a minute. i had my period this month at the normal time and it ended at the normal time but a couple days after it ended i started spot brown blood it only lasted a day and i didn’t think much of it but i just went pee and now i’m spotting light pink. i also have cramping and i’m sexually active. i know the chance of actually getting pregnant on nexplanon is slim but i’m still worried :(