Am I in labor? Help!?

I'm 18 weeks pregnant with twins and I think I'm in labor. I am feeling this severe pain in my back and the top of my abdomen, and it feels like I have the runs coming on or something, only it's a lot worse. The pain stops for a few moments, but it always comes back! What do I do? Am I in labor? There is also some brown and bloody substance coming from my crotch! What do I do? Will the babies be okay? I have nobody here with me and I don't think I can make it down the steps to call anyone! Help!!!