I've heard about this happening before, and it's probably yeast that has, for some reason or other, decided to cling to one side of the bottle rather than head for the bottom. Two things you can do (assuming that it is yeast). First, do nothing and just enjoy the beer. Second, roll your bottles on the floor and then put them back into the fridge. That should knock the yeast back into the beer, and then it can settle out normally.
Good luck.