If it’s just sometimes, there’s nothing wrong. There’s a lot happening in your body and your head when you’re young. If you want to be left alone, then be alone. And it’s not such a daft idea to turn off your mobile occasionally – more people should!
Is it normal to just want to shut yourself in your room and stare at the ceiling all evening long when you get home from school? Sometimes, that’s all I want to do. I even turn my mobile off.