Imagine going to your mailbox and getting a letter from a collection agency regarding a medical bill that you don't recognize, and that was incurred in a state where you don't live. Also troubling: The collection agency has your Social Security number, but it's definitely not your bill.

A reader named Sharon faced exactly that situation and wanted to know what to do -- and how the agency could have gotten her Social Security number.

First, it's important to make sure the bill is not yours. It's possible the number was a simple typo, or that her own doctor used a lab or other service outside the state, and she didn't recognize the charge. You should check with your health insurance company or Medicare to see if a claim was filed.

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If, after checking, you're certain the bill is not yours, you should write to the debt collector.

"I recommend sending the collection agency a certified letter stating that you don't believe this bill is yours, and asking for proof of the debt," says Gerri Detweiler, director of consumer education for Credit.com. (Certified mail gives you proof of mailing and also guarantees delivery to recipient.) Under the Fair Debt Collection Practices Act, you have the right to request verification of the debt, and you may also ask the agency not to contact you again, even though that doesn't mean the debt will go away.

As for how the collection agency got Sharon's Social Security number, it's impossible to know without further investigation.

It might indicate that Sharon is a victim of medical ID theft. Regardless of how it happened, it would be a good idea to check her credit reports, which are also linked to your Social Security number. She can get a free report once a year from each of the three major credit reporting agencies, and she may be entitled to additional ones if it appears she is a victim of fraud.

If she should find an error -- a distinct possibility, given that the bill in collections is associated with her Social Security number -- she should dispute it. That way, she can keep her own credit from suffering because of the collection action. She should also watch for credit score changes by using a free tool like Credit.com's Credit Report Card, which updates users' credit scores every month.

Image: Monkey Business