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With females, fortunately, their anatomy won't allow them to totally block, so what is happening in your girl's case is that she does have a urinary tract infection. The problem is that she "thinks" she has to go because of the irritation in the urinary tract and/or bladder, but her bladder is truly empty. (If you've ever had a urinary tract infection, possibly you'll understand the feeling she has).
Definitely get her into the vet tomorrow. They'll put her on antibiotics and that should clear up the problem. If this was a male cat, I would tell you to take him to the emergency clinic, because they CAN develop total blockages and the bladder will rupture, but your girl will be okay until tomorrow.