some batteries are just dodgy. Mine was good until recently 05 - 10 not bad really but my brothers bike the battery is strong as ever we bought our bikes on the same day. Auto zone has them for $60.00. yours could be still under warranty though. You could hook a multimeter up to it while running and see what kind of voltage the alternator is putting out.
Its probablyu the battery though. It happens

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe