It sounds like she's not happy with something Pixie. It can drive you nuts (what with all the extra washing etc.).

Can you lock the cats in during the day (whilst you're not about) as it may make her feel more secure. This of course only works if you supply a litter tray and none of the cats take it upon themselves to take the catflap apart......... Bless 'em....

There's a spray called feliwell which does work (short term), but it's expensive and doesn't help if the cat persists in being stressed out.

As for the neutering of cats stopping them spray - yes it is a myth, both my cats (one male one female) have sprayed or widdled at times of stress (namely a couple of big cats kept coming into our flat) and both have been neutered since they were kittens.