Robb seems to have sprained his ankle, some time during our kitchen-painting project. Since he can't really feel his feet and legs, it's hard to notice when he's injured himself. And since his limbs don't always behave the same way, from day to day, it's also a challenge to decide of there's actually something wrong with them.
What Robb noticed was that one of his ankles was rather swollen. Not the pain, because there wasn't any. A similar thing happened when he broke one of his toes. He was alerted by the bruising, because he didn't feel any pain at all.
I don't think Robb has sprained an ankle previously. He's trying to keep the leg elevated, and to keep off of it as much as possible. Knowing Robb, he'll probably build some crazy contraptions, to make his life easier. And he'll train the cats to brew and carry his coffee.