What Causes Vaginal Spotting Before A Period?

Hello, I am here because my girlfriend told me she had light pink blood when she wiped after urinating. She is due for her period any day now because it's been about a month. She told me when she went to the bathroom again it was clear. We had protected sex a couple days after her last period finished and the condom we used was not broken at all from what I observed. From what I have read, implantation bleeding occurs before a period is actually due, usually 6-12 days after ovulation. Is implantation bleeding possible for her, or is it likely just the start of her period soon?