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's happening: Stock in Westport Innovations (NASDAQ:WPRT) is up big again, with Monday's 15% gain putting it up almost 70% since Feb. 1:

WPRT Chart

WPRT data by YCharts

Why it's happening: The thing is, there really hasn't been any big news out there in weeks. Yes, there have been a few press releases about Westport's WiNG version of Ford's Transit van receiving EPA certification, but Westport released that news more than one month ago. And while there's some potential that the steadily rising price is the result of short-sellers exiting their positions, even this isn't likely the only explanation. After all, shares sold short have been on the decline for more than a year:

WPRT Percent of Shares Outstanding Short Chart

WPRT Percent of Shares Outstanding Short data by YCharts.

When there isn't one or two things to point out, it's usually a combination of factors behind big price moves like this, and that's likely the case with Westport's stock. Beyond the short positions, there's a good chance that some momentum is building right now. The stock was viewed by many as "oversold" at its lows, and there has been some positive macroeconomic news, with U.S. jobless claims down and overall employment up. Furthermore, crude oil prices continue to slowly climb, and it looks like the latest news on oil rigs -- oil rigs in operation have fallen 20% since last year -- point toward oil prices continuing to rise going forward. 

Yet even with all the good news, Westport remains a high-risk investment. Its core business is still in Europe and Asia, and economic uncertainty there -- combined with the company's continuing losses and shrinking cash reserves -- are no promise that the recent gains are sustainable. While the stock probably was oversold, don't confuse that as evidence that it will keep going up in the short term. You can, however, guarantee that big one-day moves probably will continue -- but they could also be big moves down. Only investors willing to ride out that volatility need apply.