You were Bellatrix Lestrange's daughter, and your best friend was Mattheo Riddle, Voldemort's son so both of you were good on terms. Mattheo secretly likes you, but doesn't show it, though he has seen you making out with a lot of boys, and he was jealous but doesn't show it. It was a nice potions class, and when the bell rang you stood up and walked away without waiting for Mattheo. So when Mattheo notices this he goes to follow you.