It could be hacked (there could be other reasons for all that's happening too, but the cure would be the same). If you reflash the ROM you'll get rid of anything that didn't come with it when it was new. See
[Samsung] How to flash Stock ROM via ODIN. You'll lose all your data, pictures, etc. If the phone is hacked, you don't know which files are involved, so backing them up, then restoring them might not fix anything. If you don't have a backup from before it all started, you should have been doing incremental backups, so I guess you'll start now.