Get her checked out by a vet asap.If she isn't pregnant she may be having a "Phantom pregnancy".Im not exactly sure what causes it but it can lead to some quite severe effects for the poor thing!!!!.The vet may advise you to get her x-rayed to prove that that there is nothing untaward in her womb.You may also be advised to get her spayed if you dont want her to have any kiddies in the future.
A phantom pregnancy is usually caused by a dog going into season and out again without mating; they just seem to decided that they're pregnant anyway! They tend to last a couple of months (the usual gestation period).