When I look at the storage section of my phone, the various categories add up to about 2.5 GB of storage, but it says that 11.4 GB is being used. Is there any way to figure out what else is taking up space on my phone?
Please check out What is other on iPhone Storage to get a nice way in finding your hidden storage waste.
Also to give you a nice way to fix this. Restore your iPhone to the most recent version and then place the backup file back. Don't just update as it won't remove anything.
Also check out this Apple KB about the space issue.
To summarize Apple's solution: