Fears of a post-holiday retail hangover may come to fruition this month, as consumers begin receiving bills for purchases made in November and December. According to a report released this week by America's Research Group, more than 43 percent of shoppers in the U.S. exceeded their holiday spending plans, and one-third of shoppers used credit cards - up from 16 percent in 2010. Britt Beemer, CEO of ARG, pointed out that post-holiday spending may plunge to some of the lowest levels in years. "Americans now face the challenge of bigger credit card debt until they get their income tax refunds - but, regardless, spending levels will drop for at least the month of January as the sticker shock hits big-time," Beemer said in a statement, adding that spending may hold off for the next three months, unless considerable savings opportunities emerge. Given consumer spending accounts for more than two-thirds of the U.S. economy, a slowdown in activity would be disastrous considering the fragile state of the economic recovery. Furthermore, retailers may suffer weaker profits as a result of steeper markdowns in the weeks leading up to Christmas.