What Causes Pink And Brown Vaginal Bleeding?

Last month I had been taking my pills sort of late. i had sex once but I took my pill at the right time that day. I always get my "period" the same say, the 3rd sugar pill. I did get my withdrawal bleed on that exact day but I felt like maybe it was a bit less than usual, but I also had been worrying so I may have been paranoid.
This month I have not had sex. I have been taking my pills much later than usual and took two the same day. I never had breakthrough bleeding before and I am not sure if that is what this was: I had a pinkish lightbrownish tinged discharge that was not mucousy at all. It only happened once that day and my discharge is normal now and that discharge hasn't happened again. Was it breakthrough bleeding? Could I be pregant even if I had my withdrawal bleeding on the exact same day I always do? Please help :/ I have not been able to get any answers and I cannot ask anybody I know.