Battery Life Issues!!

[emmajean15]

There's something up with the battery life of my iPhone 4S (16GB). I purchased it brand-new in June of 2012, and it worked like a dream up until a few months ago. I could charge my phone with the wall charger until I had 100% battery, and then use the phone normally, but the phone would not make it to low battery before the phone died; sometimes, my phone would die even around 45% battery life. It only does this for short periods of time, though; after about a week of my phone dying on ~40% battery, it will revert back to making it to 20% or 10% battery life before dying. Could someone explain to me why this would happen, and if there's something that I'm possibly doing wrong? I'm not a very tech-savvy person, and any advice that I can get would be very welcome!