I have ankylosing spondylitis that has caused all kinds of problems from my eyes to my toes, but now something is happening in my feet that is unbearably excruciating. Following a recent flare-up of joint pain all over, I started getting horrible pain on the back of my feet, above/behind the heel and below the ankle (the back of the "ball"). It's very sore and tender to the touch, and it gets sharp when I flex my feet, like pushing off the ground while walking. The only way I can walk is to waddle flat footed. Using stairs is very difficult (both up and down), and I have to do it one stair at a time using the railing to pull my body because I can't push with my feet. I've had plantar fasciitis, and I recall that pain being more on the bottom of the heel, as opposed to this pain which is on the back of the heel. The pain radiates a little bit around the outside/bottom of the feet as well, but the bottom of the heel has no pain. It started about a month ago, and it does not seem to be getting better. As a someone who lives with a chronic pain condition, I can handle quite a lot, but this has had me home-ridden for the past month. I cannot put on shoes, and I'm walking with crutches. It's ruining my life.

Can someone please tell me what this is or what's causing it?