At 13w6d I had a few large gushes of bright red blood. I thought the worst and went to the ER. Baby was fine, cervix was closed. After multiple appointments, doctors, and ultrasounds, I was diagnosed with a partial placental abruption and a large clot in my uterus. I stopped bleeding the first night but spotted brown for 6 whole weeks, sometimes a lot of brown. It's been incredibly nerve-wracking, but the baby is still fine. She's growing very well so can surmise that the placenta is fully functional. As of earlier this week, the doc cannot locate the abruption or the clot.(Great news!)
Blood is always scary. I'm glad your doc isn't worried, but if you still are, call him/her and schedule an appointment.