You need to see your vet. It's a sign of infection (maybe urinary tract issues which they are prone to along with a couple of the following) and things like diabetes and thyroid problems or even kidney trouble and other things. Your vet will be able to narrow things down a bit once you go in and they do an exam and get history and talk to you about everything. Good Luck to you.
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www.americanshihtzuclub.org
www.dogfoodanalysis.com
www.bornfreeusa.org