you may want to try resetting your power management unit:
1. If the computer is on, turn it off.
2. Reset the power manager by simultaneously pressing and then releasing Shift-Control-Option-Power on the keyboard. Do not press the fn (Function) key while using this combination of keystrokes.
3. Wait 5 seconds.
4. Press the Power button to restart the computer.
you may also want to check you battery's capacity. you can download capacity meter from versiontracker.com and it will tell you how much juice your battery is capable of holding.