Hi Adelaide, it’s hard to confirm without observing your workouts or more nutritional details, but by the sounds of things your added shakes are doing the trick. When muscles have been worked hard, they need to be adequately fuelled with protein in under 2 hours after the workout to repair and rebuild effectively. Whey protein is especially effective because it is a fast-absorbing protein compared to other sources, like meat etc, and including it into your shake could certainly have helped your muscles recover quicker than normal, hence the diminishing of pain the next day. A good rule of thumb to ensure that you’re lifting heavy enough is to aim for 3 sets of 8-12 reps, where you can only just complete the last few reps.