9 months breastfeeding, deflated boob?

Title pretty much says it all. I've been BF for about nine months, and just this week I've noticed one of my boobs is like completely deflated. I'm still nursing on that side, and can feel a let down, but it takes a lot longer. It also never feels full, which I know isn't a sign of less milk, but still so odd. The other boob is giant compared to it. I expected this to happen when I stop nursing, but I'm still feeding him on that side regularly so very confused.