HELP* MBP Auto Shutdown

HELP* as the title said..
My MBP went straight to shut down mode without going to the sleep mode where it should've when the batt is running low or near flat
Anyone knows what's going on here?
Thanks...

What were you doing before this happened?
The computer will shut down automatically in two instances: no power (eg. battery is drained) or overheating.

Just some usual stuffs like word, safari, itunes
It even went off when the batt indicator shows 7%
I recalled even when its flat.. it will go to sleep mode and save up your work before eventually shut down..
This time round.. it just shut down.. without even saving my work..
I hope its clear =p