I Am 17 Years Old. I Have Been Sexually Active

I am 17 years old. I have been sexually active for 2 years with the same person. I have always had regular periods, and I am on birth control. Two days ago, I woke up, went to the bathroom and noticed I was bleeding, like a period. The blood was light, almost pink, and I had lots of dischsrge along with it. I had just gotten off of my period a week prior to this day. The bleeding only lasted a day, and I never had any on my underwear, only when I wiped. Why was I bleeding?