my usage says my phone is full, but i deleted almost all aps, messages, pics, movies, music and it won't change

Today as i was taking a picture, my phone told me that my storage was almost full. This often happens due to me watching videos, etc, and i just delete the ones i've downloaded and sometimes pictures and everything returns to normal. Today, i have no videos and minimal pictures. i added up the usage from the apps and it comes nowhere near close to the amount it says i'm using. is something wrong with my phone? Please help. i've shut the phone down and restarted it several times. i know you can reset it, but does that mean i lose everything?

Any help would be appreciated. I have the 4S by the way. hoping for a 5 for Mother's Day!

Kelly