I too would suggest that you take her to the vet and ask the vet to do a needle draw from her bladder and test the urine. It could be that even though she doesn't "seem" sick, she is. They hide it so well, they truly do. Once you have ruled out health issues, you need to look at behavior and environmental issues. But first, rule out the health end of it. Stress can cause a cat to get sick quite quickly. And vet visits are stressful for any cat.
Also have you seen our post here?http://www.thecatsite.com/forums/showthread.php?t=9563
You will need a good enzyme cleaner to take the urine smell and stain out Nature's Miracle or Nok-Out might work, but you would have needed to apply those while the urine stains were fresh so it may not take all the old stains out. Also, I hope you didn't punish your cat for peeing on your bed- she more than likely couldn't help it.
Also I read you other posting about how you didn't get help. The board crashed recently and we lost track of a lot of important threads. Trying to get back from that has been difficult. I saw your subject line Stella got spayed- and figured (wrongly) that it was about a spay operation and you were worried while she was in the vet's office having it. Nothing in your subject line alerted me that there might be a problem, so I passed it off and didn't read the post and for that I apologize-