Firstly, don't worry. It happens to an awful lot of bitches. Unfortunately it can get worse as she gets older. Having a litter does NOT help them go away in the future!
I always take away the bitches toys, give her less food and water, and plenty more excercise. ;) this helps to get her mind away from the phantom. If she produces a lot of milk, or it goes on for a long time, you can get something from the vet to help reduce it.