Mine did this once (had it 3 mos.); I took it off the charger, drove for 45 mins. (not using the phone at all) and when I pulled it out, the LED was blinking red. Uh-oh. I had the charger, so I plugged it in for a couple mins. and it went back to 100%. I'm crossing my fingers it doesn't happen again.
I'm running stock with about 70 apps, but nothing new added near then.