Retailers reported robust numbers on Black Friday last year, which surprised some analysts because the economy is still struggling and unemployment is high. But new data from the Federal Reserve shows where much of that money came from: credit cards.

The report shows consumer borrowing surged by $20.4 billion in November — the highest month-over-month increase in a decade — bringing Americans’ total credit card debt up $798 billion. And that number doesn’t even include what we spent on holiday shopping sprees in December.

While November’s figures are still slightly lower than the approximately $800 billion in credit card debt we carried at the end of last year, some experts are worried about our ability to pay it off.

Curtis Arnold, founder of the site CardRatings.com, says the growth of aggressive offers from credit card issuers that promise zero percent APR for balance transfers or purchases is one reason for all the new charges, but warns, “You can ride those offers for only so long.”

When the teaser rate expires, some consumers could be in trouble. And if the economy takes a turn for the worse and more people become delinquent on payments, issuers could curtail those offers — so if you’re planning to roll your new debt into yet another zero percent offer, that idea may prove futile.

“The telling thing about this is borrowing is really up, but at the same time our savings rate is down and our income growth is not even keeping up with inflation,” Arnold says.

His advice? Pay off those charges before the teaser rate expires — or risk having high-interest debt later on.

[Time]