When i swallow there's something white that comes out of the back of my throat. and goes back down. it's not on my tonsils. what can it be?

Uvula: The only normal structure in that region which would move that much on swallowing is the uvula which extends downward from the middle of the soft palate in the back of the throat. It is usually more of a pink color. Have your physician examine your throat if you still are concerned.

Epiglottis: May be what you are seeing is epiglottis a small structure located behind the tongue that covers the trachea when you swallow . It is a normal structure. I am surprised how can you see it when you swallow. You can see it when you stick out your tongue. If you are concerned get it seen by the physician.