What Causes Vaginal Bleeding Between Periods?

Hi I am 24 years old I had my pd 8 days ago on the 21st of December-27 of December but this past Friday on the 2nd of January I experienced some light bleeding after that I have been spotting light brown and light pink on my underwear or when I wipe. By the way I have been sexually active with my husband since I ended my pd what could this mean? Please help