It's quite likely you have a bad battery. From the Apple menu, choose
About This Mac and then click the More Info button in the resulting
window. When System Profiler appears, select the Power entry and
eyeball the Health Information entry. If, under Condition, it says
"Check Battery," that battery is probably on the fritz.
Fortunately, these batteries are under an extended replacement program
as they're defective. Apple has issued a document that outlines
symptoms a defective battery might exhibit as well as a link to a
software update that can fix some battery issues.
http://www.apple.com/support/macbook_macbookpro/batteryupdate/ If the software fix doesn't work, you should follow the document's
advice and contact Apple about having the battery replaced.