You can get by with so much when you are young, right? As we get older, we have to make some accommodations. And one of them is with our sleeping position. I learned this from my own experience. As we get older, sleeping on our side can cause our hips and/or knees to hurt, especially women because our hips are wide.
I bought a pillow especially made to put between my knees while on my side. It was about 10 inches in length. What a misstate that was! My knees hurt more than before. I had to experiment. I found that I feel best when the pillow I put between my knees goes all the way from the top of my knees to the bottom of my feet. Since I am short, I can use a regular pillow. Why is that better? The short pillow left my leg hanging especially at the knee and ankle. For a minute that is fine, but you sleep in the same position sometimes for hours. It pulls on the ligaments. Try using a pillow between your legs that goes from your knees and is long enough to still supports your ankle. See if that doesn’t make a big difference for you like it did me!