when you’re sad for no reason

I hate when you’re sad for no reason; you’re with people, you’re fine, you’re happy, you’re laughing and smiling, but at the same time it almost feels like you’re not there. You keep dazing in and out of conversations, you cant focus on one single thing. And once you’re by yourself you don’t want to do anything, you’re sad and feel alone. Someone asks you what’s wrong and you want to tell them, but you don’t even know what’s wrong yourself.