Just because your credit card has a fixed rate today doesn't mean it always will.

Along with changing your interest rate, card issuers can change the way your rate is calculated, says John Ulzheimer, president of consumer education at SmartCredit.com. So, a card that carries a fixed rate could become a variable-rate card in the future, he says.

If the interest rate goes up because of the change, then the issuer must give 45 days' notice under the Credit Card Accountability, Responsibility and Disclosure Act, or CARD Act, says Ulzheimer. If the rate stays the same or decreases, it's unclear whether the issuer must tell you.

But if the issuer wants to slice your credit limit or close your card, it doesn't have to tell you until after the fact, he says.

On the upside, the issuer must send a copy of the credit score used to make the decision to lower your limit or close your account under new federal rules issued last year.

Also, if you're declined, the bureau which provided the information must give you a free copy of your credit report if you ask, says Ulzheimer.

"At least you know why," he says. "Before, it was kind of a mystery and you really wouldn't know."