What is wrong with my fish?

My fish have been acting strangely for a few days now (I've had them two weeks). One minute they will be lying still on the bottom of the tank, the next swimming round completely normally. There are two fish, both are oranda goldfish - one calico and one chocolate. Neither are mature yet. The tank (if it helps) is about 14 liters, and came as a National Geographic "set up a fish tank" set, in which the filter (a standard Elite) was also included. Please help!