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 retailer Express (Nasdaq: EXPR) were looking pretty unfashionable today as investors knocked them down 13% after the company's earnings announcement.

So what: The numbers from the first quarter actually looked pretty good. Comparable store sales growth of 8% drove a 10% increase in overall revenue, while operating income jumped 35% as the company's operating margin expanded from 12% to 14.9%. Sales of $467 million and adjusted earnings per share of $0.42 topped the respective average analyst estimates of $459 million and $0.40.

Now what: Obviously investors were reacting to something other than the first-quarter results, and that something may be the company's guidance. Management projected second-quarter EPS to be in a range of $0.12 to $0.15, while full-year EPS will likely be between $1.52 and $1.61. The full-year range is actually a slight increase from the last projection that the company offered, but it appears it wasn't enough to appease investors. Wall Street's estimates for the quarter and full year were $0.16 and $1.58, respectively.

Want to keep up to date on Express? Add it to your watchlist.

Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool’s disclosure policy prefers dividends over a sharp stick in the eye.