Shares of Aeropostale Inc. dropped Thursday, after the company reported surprisingly weak sales in stores open at least one year in October and an analyst downgraded the stock.

Aeropostale has gained market share during the recession as consumers hunted for bargains and stayed away from higher-priced teen apparel stores like Abercrombie & Fitch Co.

But Aeropostale said sales in stores open at least one year rose just 3 percent in October, widely missing the 13.8 percent increase forecast by analysts polled by Thomson Reuters, on average.

Shares fell $5.39, or 14.2 percent, to $32.64 in afternoon trading. The stock has traded between $12.52 and $44.85 over the past year.

BMO Capital Markets analyst John Morris downgraded the company and said it is likely starting to lose market share to competitors.

"The company reported significantly disappointing October (sales of stores open at least one year) of 3 percent, which leads us to conclude that Aeropostale is at the beginning of a market share-losing cycle," he wrote. "This factor combined with our view that Aeropostale is coming off historically high sales productivity and margin levels leaves us with the outlook that earnings per share growth is likely to decelerate significantly next year."

He downgraded the stock to "Market Perform" from "Outperform" and slashed his yearly earnings estimate to $3.20 from $3.65. Analysts, on average, predict a full-year profit of $3.16 per share.