Do take her to the vets we had this with a cat and it was a urine infection, more common with Toms, but females suffer too. We left it far too long before realising anything was wrong thinking it was the cat playing up!
Oh gosh, that's not good. You already have trays for her - two is great and you've ruled out a urine infection, so I'm a bit stumped. As you may have noticed in 'the litter tray' whenever possible stress is mentioned we always recommend trying Feliway plug-in, and that would be worth a try in this case. Any biological washing liquid/powder and at least a 40 degree wash should get rid of the smell completely. I suppose the obvious thing to do, unless you live in a bedsit, is just keep her out of your bedroom for a while?
We have a new rescue cat who is 7 months old. She has started weeing on my bed and it's driving me mad. I've got a few theories or tricks to try to stop it but any advice would be good. I don't think it's a urine infection, she might be a little stressed having moved to us but when she arrived she started off using her litter perfectly and doesn't show any other signs of stress. Her tray is cleaned regularly and I have two trays in case she goes in one. Any ideas? Also what's good to wash my laundry with to remove her scent? Thanks.