Storage Problems.

I’m using an iPhone 6s that has 16GB of storage. It always tells me that I’ve run out of space and I should manage my storage. When I go to check and actually count the amount of space each app has used , it doesn’t even add up to 16GB. It adds up to about 11GB. My notes are apparently using 6,3GB but it’s really unrealistic because i don’t even have many notes, yesterday deleted notes from last year backed themselves up. i can’t even access Whatsapp or other important apps on my phone because of this because I don’t know what the problem is , Can you help me?