Battery indicator light blinks red

I was talking on my cell phone and the battery ran down (it had one bar left and the indicator light was yellow). So, I plugged it in to the wall while talking. When I looked again, the battery indicator was now blinking red. What does this mean? If I unplug it, the battery appears to be charged a little bit but quickly dies again. This phone is new and I couldn't find anything about this in the manual.