Since I've started riding (years prior) it seems I've been having problems with my left knee. It's prone to lock up, and recently, it swelled several times it's normal size and was very painful to walk on. Now I have my range of motion back and can walk on it again thankfully.
Recently I started riding again, western. Whenever I have my feet in the (hopefully) correct position, the outside of my ankle hurts like h***. When I dismount, my knee is usually sore until I walk on it a bit.
I keep my feet with heels down, toes up, with the balls of my feet in the stirrup. It just hurts, and only on the one side. Drives me nuts. I try to keep the weight off the stirrup, and have a deep, light seat using the upper leg. Back is straight but not rigid (I can't do rigid, or to inwardly curved due to a past injury, heh).
Perhaps I'm doing it wrong...I dunno, that's how I was taught >-<