I agree with Lucky. If it's not a uti or bladder stones, then it's spay incontinence. My doggy has that and it costs me about $30/year (although the meds are increasing in price these days). If it is that, you'll either get hot pink estrogen pills to give her (they come running when you shake the bottle) or a liquid to put in her food that strengthens her bladder muscles.
Good luck and please see your vet. You don't have to live in frustration.