My knee feels like it s being squeezed in a vice, it can happen after I ve been sitting or standing, with or without pressure, generally just after it s been in the same position for over 5 minutes or so, when I move it, I.e stretch out my leg to straighten it, or bend it if my legs been straight I get excruciating pain that lasts for a couple of minutes and causes me to limp until the pain goes, what s causing it and what can I do to stop it?