What Does Light Bleeding During Periods Post Having Unprotected Sex Suggest?

HI, I am in need of some advice of what I need to do. I have had regular mestrual cycles the last 11 months, usually 28-30 days apart. The first day of my last period was 12/6. I was due to start 1/4 or 1/5. I started spotting on 12/31 a very light pink. Later that night it turned into bright red. The day of New Years, I was bleeding bright red all day. Not heavy as my regular cycle. Since the 2nd, I have only had very little brown spotting. I have taken several home pregnancy test and all show negative. I am so confused on why I would have regular cycles then start 5 days early and only bleed for one day. Can someone please shed some light on my situation? * Husband and I had unprotected sex on 12/22. Thank you!