I found this too. You've probably knocked your wheel balance(s) off. Even if you haven't lost a weight, its likely that the extra snow/ice in the wheels makes them unbalanced. Try to clean the snow and ice off (warm water is best, since it gets behind the wheel, and gets all that off too). Note that you need to do this in a way it won't refreeze, such as a garage until they are dry again.
