Bleeding

This morning I woke up to pink blood, went back to sleep and when I woke up again I had quite a bit of dark/brown blood and passed a clot the size of two quarters. I called my OB right away and they got me in to an ultrasound. Baby is totally fine with a strong heart beat, thank God. Has anyone experienced this? I've never had any bleeding with any of my previous pregnancies. Thanks for the help!