In car the seatbelt jolted it was sort of accident and my shoulder is swollen and red and can't touch it what should i do mum says it's only bruised

First test mobility: Move your shoulder by raising your arm, extending it out, throwing motion, see if you can lift a light weight(a pound or so) to assess range of motion. In otherwise healthy person, seat belt should not cause major injury. Those movements, if done easily, indicate that shoulder is probably ok. If not, or there is severe pain, it should be examined by physician.