iPod Mini Battery Icon with Lightning--PLEASE HELP!!

When I turn off my iPod, the battery symbol shows up on the screen with the lightning bolt on the left of it--even though it's fully charged. When I leave like that for a period of time and come back to it--the screen is blank but the iPod won't turn on again until I plug it in and charge it again. I think that symbol showing up is running down the battery or something.

This all started when I plugged my iPod into a different computer and I guess it got "confused" and shut down and I couldn't get it to come back on. However, I plugged it into my car charger and reset and it seemed to work fine except the battery icon in the corner had that lightning bolt.

It works fine otherwise it just seems like I will have to have it constantly charging instead of letting it sleep?!
Any advice?