A cracked filling on one of my teeth?

Okay, so today, a part of my filling came out (If it's relevant, it was on one of my back molars). The spot where the filling came out though had been a bit sore whenever I chew on something since I got the filling, which was about two or three months ago. My brother checked it out and he said it looked cracked, but there was this sort of brown spot where the crack is. Is this normal?