You've probably been seen by your OB by now, but I just wanted to let you know that I had my 12 week scan yesterday and they saw a small contained bleed behind my pregnancy. It was little a little fluid filled space. They said it wont cause any problems but could come out at any time. You could have had something similar and with bubs moving around so much, he/she could have released the blood? Apparently its quite common and no dangerous at all.
I hope all is well.