LG-P990 (optimus 2x) with CyanogenMod 7 turns off suddenly and battery indicator goes straight from 100% to 0%

I installed CM7 on my LG P990 five months ago and it worked well. Yesterday, I was watching a video on YouTube and my phone turned off suddenly. It didn't want to turn on, and then my battery dropped to 0%, even though it had been at 100% a minute earlier. I connected my P990 to a charger, and then my battery percentage indicator was at 100% again. Why? I hate when my phone turns off and black screen of death until I plug a charger in! I did a data wipe/hard reset the but problem is still there.