Hi hon! Ang dating mo in December ay parang breakthrough bleeding lang dahil sa activity, kaya ang January bleeding mo ang counting mo as your actual period, so that January 3 spotting onwards, that's your cycle reset na, count mo yan as Day 1. Since you had activity between cycles like that, irregular bleeding happens talaga, pero kung worried ka pa rin about pregnancy or if your January period feels super different from normal, best to do a test or check with your OB para makapag-relax ka ng maayos. A few of us had similar confusion before and ang clarity lang from the doc talaga ang nakatulong sa peace of mind!