my iphone 3gs screen has changed colour

hi just wondering if any one has an idea whats up with my iphone it was working fine went to bed got up and the screen had changed colour so my apps and web pages display is red and bright blue was on and off like this for a week or so but if i locked an re-opened it woud go back to normal but now for the pas few days it stays stuck like this and is driving me mad!! i can use it fine but cant play games or anything as the only colours it displays are red, blue and black Help please 😉