ET obstruction can be due to various reasons like upper respiratory tract infections

ET obstruction can be due to various reasons like upper respiratory tract infections, adenoiditis, cleft palate.

Once tube gets blocked, there is no air movement to and fro from middle ear.

Already present air gets absorbed into surrounding blood vessels in middle ear.

So, no air means no pressure inside, hence negative pressure develops and tympanic membrane gets retracted inside 

further transudate from surrounding blood vessels drawn initially into middle ear cavity and there will be pain due to retraction of tympanic membrane,

this condition is called acute otitis media.