Well, then, you have your answer: not enough boxes, not clean enough, possibly a litter she doesn't care for. "Relatively clean" is not acceptable, certainly not with 8 cats.
Here's the test: you stick your nose 4" above the litter, take a deep whiff, and dig in it with your bare hands. Don't want to do that? Neither does Cleo!! And she is doing her very best to let you know...are you getting the message?
If fixing the litterbox situation does not work, then a vet visit is imperative to rule out urinary tract inflammation.
I really urge you to read through the articles at http://www.littlebigcat.com/?action=library
. They're free, all you have to lose is a little time, but you could gain a great deal of insight along with practical suggestions for solving the problem.