Do you have messages with a lot of videos in them? Do you have a lot of email messages with videos or photos stored on your device? Either of those could take up a lot of memory.
Have you tried resetting the device to see if that will free anything up? If not, then Hold down the Home and Power buttons at the same time and continue to hold them down until the Apple appears (up to 30 seconds - really....). Once the Slide to Unlock screen displays, check to see if the usage looks a little more in line with what you are expecting.
You can also back up your device to iCloud or iTunes, and the go to Settings>General>Reset>Erase All Content & Settings. Set up the phone as new the first time around and see if the usage looks as is should. If it does not, then I would recommend taking it to Apple to have it looked at. If it does, then Erase it again and then restore it to the backup you just made. See if things look OK at that point. If they do not, then you may have corrupted data on your device,and you should set it up as new.
Best of luck,
GB