Help me!!

I don't know whats wrong with me. All I know is everytime i'm in school I'm happy. But after i come home my mood swings. I feel sad and lonely. i don't feel like talking to anyone. I have a huge headache now also. My parents get mad at me saying that i'm always angry. Well i'm not angry. I'm just sad and i don't wanna show it to them. i guess thats the reason why i look like i'm angry.
I don't know what to do. I don't know who to talk to. My parents are asking me whats wrong. I don't know what i should tell them.