brown blood at 19 weeks

I'm 19 weeks today and I when I got up I noticed dark brown blood. It wasn't light but it wasn't heavy either. After about 15 minutes it got lighter. I got freaked out and went to the er and they did blood and urine test but they couldn't figure out what is wrong. They did an ultrasound and everything looked great and I was even measuring 4 days ahead. Does anyone know why this could be happening?