B.B.
Please don't freak out!! I know, easier said than done. But I was exactly where you are a few months ago. And I am now almost 7 months pregnant with out ANY complications!
Right around the same time as you (10 weeks) I bled so heavily that I thought for sure I was having a miscarriage. My husband and I mourned the entire weekend. On Monday when we finally got into my OBs office, there was the little heart beat w/o any signs of anything being wrong. I went on to bleed for over a month...no lie. I bled every single day for over a month - red and brown. I can't even tell you how many u/s I had, no one could ever find a "reason". Once I hit my second trimester it just stopped one day. And I have been fine ever since.
I hope you have the same outcome as me!! The fact that you already saw the heartbeat and you are 10 weeks is a VERY good sign that this is just one of those whacky pregnancy things w/o any explanation.