I have only used 7GB on my iphone 5S but it says my memory is full all the time?

My iphone 5s should have 12GB capacity yet it consistently tells me I have no storage space left and I have only used 7GB??? It is so frustrating because my iphone 4 could store all my music and thousands of photos no problem yet now I have about 800 photos it is saying I have no room? I have tried deleting all of my apps and extra data but still it says I have no storage left? 😢