I would suggest trying to collect a urine sample and take it to your local vet to see if she may have a bladder infection. Sometimes when cats are urinating in odd places, it can be caused by this. Another thing that may be an issue is if your litter is perfumed or if she disagrees with the texture of it. Alternately, there is a product on the market called "feliway". It is a feline pheremone that is supposed to calm a stressed cat. Just because she gets on with your other animals doesn't mean she might not be stressed. I used the feliway with my kitties for awhile after locating to a new home. They did well with it. You can't smell it.
Worst case scenario, contact your local vet for more ideas on what the cause could be or possible ways to resolve her behavior before it becomes a permenant behavior issue.
Best of luck and don't forget to give her lots of cuddles! :)