What's Wrong With Dollar Tree Stock?

Since September, shares of Dollar Tree have fallen by 20%.Dollar Tree (DLTR 5.52%) stock has been doing fairly well this year, up 17% entering trading this week, which is better than the S&P 500's gain of 11%. But in recent weeks, Dollar Tree's stock has been in a tailspin, reaching levels it hasn't been at in months. It recently was down about 26% from its high of $118.06.What's behind the stock's sharp sell-off, and could there be more trouble ahead for Dollar Tree investors? Here's what you need to know ...