Why does my Lenovo tablet die so quickly?

I have recently noticed that when I use my tablet it tends to die a lot quicker than when I first got it. I have tried decreasing the amount of apps I use on it and changed the settings for it to be more battery efficient, like turning off wifi when not using it, but the battery still drains quickly. When I plug it into the power adapter it will stay on and charge, but as soon as I unplug it, it starts to drain quickly again. What is going on with it and how should I fix it?