my shower is contantly running and I cant stop it why?

My shower has been coming on and off while I have been using other utilities in the house ie bath, washing machine and other sinks it even runs when I flush the chain!. Today after running the bath the shower is now constantly running and the only way to stop it is to turn the water off at the main tap. Please can you advise. The shower is separate to the bath but still in the same bathroom and the I am not sure of the model of shower as there is no written details with it