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 Rite Aid Corporation (NYSE:RAD) were hitting the brakes today, falling as much as 14% after lowering its guidance for fiscal 2015, which ends next February.

So what: In its monthly sales report, the drugstore chain said same-store sales rose 3.5%, but also noted that higher-than-expected drug costs were impacting the bottom line, and it now expects a per-share profit of just $0.04 against estimates of $0.08. The higher costs were due to a delay in generic drugs and a "greater-than-expected reduction in reimbursement rates." For the full year, Rite Aid lowered its EPS guidance from $0.31-$0.42 to $0.30-$0.40. Analysts had projected a per-share profit of $0.39.

Now what: Shares of Rite Aid have soared in the past year as store closures and restructuring have paid off, and the company's pharmacy sales have gotten a boost in part from Obamacare. The stock has nearly tripled from a year ago, but today's news offers a reminder that there may be a ceiling on the stock as it now has a P/E of 15 based on fiscal 2016 projections. The good news for investors is that the updated guidance won't affect the top line, which grew 2.6% in the last quarter, but with only modest growth to look forward to shares may be fully valued at this point.