When you first got your phone, did you FULLY charge it before using it? I've noticed that it may show 100%, but is still not fully charged. I can only suggest that you run the battery all the way down to 0, then charge it until the phone gives you the message that it is fully charged/remove charger. (There's an icon that appears in the left portion of the pull-down notification bar at the top. The message will also appear on the lock screen) See if that helps. Also, check in the app management settings to see what's running. If it's something you don't need at that time, stop it.