I keep having a lot of pain in the back of my ankle where my achilles tendon is. It's really stiff and sore and I can't even move it or put weight on it because it's so painful. I haven't fallen or twisted my ankle so I don't know why it's hurting so much.

It could have been a tear that has been tearing for a while and it got to the critical mass. Could have hurt it in your sleep. Could be a pinched nerve or maybe... well you get the idea. Go to a doctor. They'll see if you need to have further intervention or if you need to just put it up.