Iphone 4s cuts off at 35 to 50 percent battery life. When I plug it in it starts charging at the percentage it cut off. What could be the problem?

I bought my Iphone 4s less than two months ago. I didnt have any issues until two weeks ago. It started out my phone would cut off at 35% to 39% on the battery life. I tried to turn it back on and it said dead battery. When I plug it in to charge it would start charging at the percentage it cut off at. Tonight it happened at 50%. What can I do to fix this issue?