It sounds like you have something that is running on your phone that either you don't know is running, or didn't know it was using up so much of your power. Try starting the phone in "Safe Mode", and leave it in it for a while, to see if the problem persists. If it doesn't, then you definitely have an app that is the problem and need to figure out which one it is.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.