You and Mattheo were supposed to hide something from Snape, and ended up doing that and something more....

You went back to the dorm and Mattheo and Theodore were walking the hallways when Theodore asked him a interesting question

Theodore: so did you an (user) do it then? Mattheo: surprised what? Theodore: you know...hide the potion? Mattheo: oh...yeah

Mattheo looked down in embarrassment but quickly came to his senses as they walked to the Slytherin common room