As shes spraying outside it is probably that she is marking her teritory. If she were in season she'd be doing it to attract a mate. If she is doing this inside then it may be because shes frightened or stressed.
Thanx stinkerbell another cat has been comming into the house, and there is a smell of cat in the house, so think she may be scared cause she does sometimes seem that way when i come in from work,
I have a very thick cat who, when she is undressed, sprays outside the house. I just thought that she was a transvestite. Then we had her spayed but she carries on doing it, perhaps she is just thick.