I try to describe the problem to our maintenance guy. I don't know what the problem is. But as you say it's more about levels (I assume).
I should have said the following, may be: 'When I'm using the sink, why is the water going up in the shower?'
But I wanted him to imagine the water going down and going up. How could I say that using going down and going up?
"The water" would imply that you think the actual water running through the sink is going directly to the shower drain. It's more likely that the water going down the sink drain is pushing some other water already in the pipes (the water from your last shower) up through the shower drain. It's better to be vague when you don't know.