Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of health-supplement retailer GNC Holdings (NYSE:GNC) fell as much as 13% today before closing down 3%, after the company reported quarterly earnings.

So what: Overall, the specialty retailer had a good quarter, beating EPS estimates by $0.03, matching revenue expectations, and raising its guidance. Revenues grew 15% to $622 million, with strong same-store sales at domestic company-owned locations, and adjusted EPS up 32.6%. GNC also announced it was expanding its Gold Card Member Pricing program to four new markets after a successful pilot program tests. The program offers members variable everyday pricing discounts.

Now what: It's odd to see shares decline after an otherwise strong earnings report, although it happens on occasion. Usually, it's an indication of other reasons that the market fears the stock. In this case, investors could be worried about recent allegations from the FDA that Monster Beverage (NASDAQ:MNST) drinks have been responsible for some deaths, or even the negative light shined on fitness supplements as a result of Lance Armstrong's fall from grace. Despite the slight pullback, investors seem to have no reason to fear here, because GNC's strong growth seems likely to continue.

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