arohap, your battery is at the end of its life.
it WILL balloon and expand inside your iphone, eventually breaking the logic board and screen.
You can easily tell if your battery is expanding just by looking at the screen and seeing any kind of pressure being placed on it, it will make the LCD crystals look all funny.
I had 2 (of my 2 3GS's that i own) both have their batteries expand and bend their logic boards with the last 3 weeks. both you could see very noticably pressure on their screens.
If you see this pressure on the screen, DO NOT Continue charging your iPhone.
my recommendation is to open up the iphone and remove the battery asap! else your logic board and screen will be destroyed from the pressure being placed on it from the expanding battery (bag).
So to finish my story, i replaced one of the batteries in one of my 3GS's just today. bought from an amazon seller, so it is not an OEM Apple battery. and now i have it charging. its notstop turning off. from starting with 4% battery upon install and now even at 18% battery, it keeps turning off. so im suspecting there has been some kind of damage done to it from the expanding battery.