Yes, sounds like she could be having a phantom pregnancy, which is fairly common and usually resolves itself. There's really no treatment (other than spaying) and generally doesn't present a problem healthwise for the bitch. The biggest concern is if she gets overly aggressive with her "baby". It's not her either, it's the hormone changes.
If your son can live without Fimbo for a little while I would suggest letting her get on with it and ignoring her, she'll get over it, usually it's very sudden too.