1/2 of all pregnancies end up in the first trimester having a threatened miscarriage according to my ob. It's just a last chance for your body to abort the baby in a sense. I was also told only 1 in 5 actually end is with a miscarriage. Take it easy, rest, drink your fluids and things should level out. I went through it and am now 17+3.
Any type of bleeding in pregnancy can be scary, its not normal but it is common, and I think with twins you have double the worry.... many things can cause bleeding that dont necessarily mean a miscarrige, I'm sure the doctor would have told you to take it easy for a couple of days if you can and also make sure your fluid (water) intake is adequate. The good thing is brown blood is old blood so as long as it stays brown thats a good sign that everything has started to settle down, good luck :)