What Causes Popping Sound In The Knee With Severe Pain On Moving? ?

Hi, may I answer your health queries right now ? Please type your query here...My knee has sometimes sort of locked or popped, from going upstairs. What happens is, I can't move it, bend it without excrusiating pain. I have to sit down and stay still for an hour or so and it eventually goes away. Once it happened from just turning to walk in a different direction. This would happen a few times a year. Lately, its been more regular...every 2-3 weeks.