Is my cat masturbating?

My cat is a 4yr old male he is neutered. He has been acting weird he acts like he's masturbating with certain pillows and blankets. At least I think he is masturbating he has always purred while kneading blankets and he always look like he is nursing but now he is doing this funny things with his back legs and tail where they will shake while he is doing this. After wards he will clean himself. The only thing I can think of why he is doing this is....We had him shaved, he is a main-coon and had knots on is tummy and this is when it started. We had him shaved once before and I didn't see him do it then although I am home now a lot more then I use to be. He still eats (to good) he drinks is using the littler box and is very playful???