What is happening is that you no longer need to do straight leg raises. Once you can do them, you don't need to keep on doing them. It's causing pain and we advise "If it hurts, don't do it." The pain is a warning that you're doing something your knee isn't ready for yet. It's still very early days - only 8 days since you had major surgery. Give your knee a break and stop doing something that hurts. Don't forget that your quads were man-handled, pulled aside and probably bruised during the surgery. They need time to heal, too. You're making excellent progress, so ease off the exercises and let your knee heal. By the way, do try not to compare your knees. No two recoveries are alike.