Spotting

I am currently 5 weeks and 4 days pregnant and I have been having 2 days of brown spotting with tiny brown clots, no cramps. Got an ultrasound done and they couldnt find anything. My hcg is still doubling... anyone know the cause of brown spotting and not being able to see the yolk sac? Last Friday my HCG level was 204 and on Tuesday it was 476.