I have only had singletons but I did have the issue with old brown blood and was told it was nothing to worry about. But like you, I worried anyway! All turned out fine with both of my pregnancies. My Dr. said that fresh blood would concern him but not brown blood.
~ Christine ~ Mom to a sweet little girl and 2 busy boys
I am carrying twins too. I had both old brown blood and new bright red blood. It scared the daylights out of me. Mine was a sub chorionic bleed that healed itself over a few weeks time. My twins are now at 20 weeks 1 day and all is well.