You may be getting charged every month for services you're not even using.

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Billions of dollars' worth of "grey charges" are reportedly hitting Americans' bank accounts each year. Making matters worse, these charges aren't illegal. In fact, you started them in the first place.

You may remember your online clothing order that came with a free 12-month magazine subscription, or that trial run of a video streaming service that was only complimentary for the first 30 days. What you didn't remember, though, was to cancel these offers before their automatic billing cycles began.

Companies are banking on your laziness and forgetfulness, and then cashing in every month or so.

"Any situation where you have to opt out before charges start, that's basically the business's way of saying, 'We're betting on you not to opt out so we can go ahead and start charging you for this service,'" said Bruce McClary, spokesman for the National Foundation for Credit Counseling.

Any free offer that requires your credit card information up front should serve as a red flag.

Perhaps the most vicious feature of these grey charges is their size, or lack of it. They don't come at you with a $65 hit that would catch your eye, but instead, $3 or $5 that's hard to miss.

"That's why it's incredibly important to look at all the details of your credit card statement every month," McClary said. "Go through it with a fine-tooth comb."

Past charges probably can't get reversed, McClary said, but the biller is likely to stop any charges going forward.

According to a 2013 analysis from Aite Group, U.S. cardholders were hit by $14.3 billion in grey charges, most of which were the product of free-to-paid offers.