Check out "Clean Master (Cleaner) - FREE"
Please only use the option to clean out junk files to start.
And personally I think that a backup is in order before going further.
Check out "Helium - App Sync and Backup"
You'll be needing to add a bit to your PC if you're not rooted but that's no big deal to get a comprehensive backup happening.
Plus, good idea to backup your pictures, etc etc, from your internal storage.
Then if that problem persists even after Clean Master, you can take your phone to a completely clean state with a factory data reset and then restore your apps and data from the backup.