Why don't you try to Check a Symptom box found on the upper left-hand side of most Health Central pages? From what I've read it could be due to things like a urinary tract infection, irritable bowel syndrome or even a hernia. You'll need to see a doctor to figure out exactly what's going on.
Here's some information on urinary tract infections http://www.healthcentral.com/incontinence/risk-factors-43-115.html
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