
A phantom pregnancy is a normal part of a bitch's hormone cycle - some have them very mildly (just act a bit subdued for a while) but others get thoroughly miserable, produce lots of milk, adopt items as substitute babies and can get very temperamental. Most are somewhere in between.
The old-fashioned treatment is to reduce their food (to discourage milk production) and increase their exercise and general busy-ness so that they don't have time to mope. Also take away any items that she believes are her puppies. There is a prescription medication called Galastop that your vet can prescribe that can help reduce severe symptoms. She can be spayed when there are absolutely no signs of hormonal activity, but not until then.