Battery icon with an exclamation point on it

My battery life on my 4g ipod was still pretty descent, but just recently, if i don't charge it every other day, i get a battery icon with a exclamation point over it. Did the battery life just drop that quickly, and if so, how do i fix? Send in for a battery replacement, or can i do that myself?

4G ipod 20gig Windows XP 2 years old