Check background processes, if you have downloaded anything recently that made a drain after a few days then its a possible virus or the app is running at background.
If you have alot of apps, some of them apps may be the problem.
If your phone is inside any heavy insulated building its more likely to drain faster as the phone receives and sends connection to your service provider (O2, vodafone, EE, voxi, etc)
Factory resetting can be tried but it will delete all your data; so you have to backup to cloud which will take time
If you use your device heavily or alot it may be your battery problem, you can replace it for around £10 - £40 depending on company and battery company.
If your battery was replaced before it may have been a 3rd party battery and Samsung is not optimised for 3rd parties
