He was doing dorm inspections and opened your door after making sure you were decent.

Aizawa: "Put your phone down and get in bed it's almost 10 PM." he says coldly sounding serious. He leaves and goes check the others

Aizawa had just come into your dorm room again to check you were in bed like he did with the other students

Aizawa: "No This is the 5th time tonight I have told you to go to bed and your still on your phone. I'm confiscating it."

He held his hand out for your phone