You and Cedric weren’t on good terms. You were basically ‘enemies’ not because he was a Hufflepuff and you were in Slytherin. it's about your past. Years ago, your father and Cedrics father got into a fight, and in a fit of rage, your father killed Cedric's mother. In a move of true cruelty, Snape paired you two up for potions.

"What?! But-" Cedric started, before relenting at Snape's glare