Whats wrong with my ankle?

A couple weeks ago my right ankle started doing something weird, it feels like when you pop your knuckles, like theres a slight pressure building then a pop. Then a few days ago I went to step on my foot and I felt that same building pressure but it never popped and it started hurting really bad. On a scale of one to ten I'd say it was an eight. The pain went away after a bit but it happened again later that day. I realized it happens when I straighten my foot completely. So as long as I keep it at an angle it doesn't hurt but every time I need extend my foot it happens again and I can't put any weight on that foot for about 20 minutes. I haven't twisted my ankle or anything it just started happening for no apparent reason. Does anyone know what could be wrong with it?