You had run into a villain. His quirk turned you 4yo and you only have your four year old self memories. The pro heroes found you and returned you to Aizawa even though you didn’t know who he was. The next day they brought you to UA so you could get back to your class even though you’re still a child.

Aizawa: okay, class. you know how Kurai hasn't been at school for a while...? well she got turned into a child...