10.5 months pumping less

My son eats breast milk 4 times a day. During the week he gets 2 bottles (5-6 oz each) and then I nurse him twice when I get home. I nurse him 4 times on weekends. In the past month I’ve noticed a significant drop in my pumping output. I pump twice at work and used to get 15 oz now I get about 10. I’ve been power pumping and added an extra pumping session but it’s making no difference. He still gets plenty of wet diapers and sleeps through the night. I was wondering if anyone else experienced this or maybe it’s a sign I need to start supplementing.