Well, if you're running your laptop with the battery in AND the plug, that could've caused some issues or if you don't use the battery enough it loses its ability to keep a charge over time. Batteries wear out faster in not completely charged or drained as well if they are used in such a state. There might've also been an issue with it from the start. I would get a new battery with your warranty while you can and then go from there. If the problem persists, you can try reloading Windows 7, or take it to Best Buy and get them to check it out.