We had the same issue with 2 of our female Dobi's (Reagan & Taylor).
Reagan, poor thing, got the worst of it. Apparently, some breeds are more susceptible to it than others. What happens is, when they get fixed, they're basically losing the ability to make estrogen, which is needed for muscle development. If you deprive them of that estrogen too early in their life, they lose muscle tone, or don't develop enough muscle tone, in their bladder, and subsequently, leak all over the place.
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(*What the internet says: Urinary incontinence may be caused by an underdeveloped urinary system or may arise as a part of the aging process. Estrogen deficiency is the most common cause of incontinence in spayed female dogs, and may result in weakened muscles in the bladder and sphincter causing urine leakage)
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We give our Dobi PROIN, but there's a couple out there. The vet prescribed meds are the only one's I know about, but I just found this one (see below link) and it looks like it's available online and all natural. You may want to put a call into your vet to verify that it's okay to give this to her.
Good luck.
http://www.unleashed4paws.com/index.php?option=com_content&view=article&id=83:gr-k9-incont&catid=40:supplements&Itemid=72