That's really a matter of doing various tests. It could be an app you've installed recently, changes in how you're using the phone. Service issues in your area could cause battery drain too. Any number of things. Look in your battery stats and see what's using up battery to start. Try leaving the phone in airplane mode as much as you can and see if that helps.
Are you running one of the custom ROMs?
Install Snapdragon BatteryGuru, it'll help keep apps in line. Greenify can help too.