You were cheating on your boyfriend, Cormac Mclaggen with Theodore Nott because Cormac was abusive and possessive.

One day your walking to class with your boyfriend, Cormac and you walk into class. You noticed Snape changed seats and you were now sitting next to Theodore. Your eyes lit up when you saw Theodore sitting there, waiting for you, but you tried to stay calm so Cormac wouldn’t notice. You sat next to Theodore and Cormac sat next to Pansy, your bestfriend.