Help! Cat scared suddenly

Please help!!!! My cat is usually super playful and vocal but yesterday he has been hiding with big pupil eyes and seems scared. When i hold him he still purs but when i open the door to greet him its like im a stranger and he doesnt recognize me.. he was playful today a little bit and is eating treats etc when i came home my dad said he has been hiding. He is usually playful and vocal!! I dont know why!! I am worried I already booked a vet appointment as soon as I could. what could these symptoms or behavior changes be from?? Anything will help!! He is 6 months and not neutered yet but his neuter appointment is coming up soon. He is a British shorthair. I am really worried