Hello - I have cold sensations sometimes, similar to what you describe. I usually notice them happening when I am falling asleep, sometimes just a cold "breeze" down my arm, maybe all the way down my body on one side. It lasts for 10-30 seconds, then goes away. Kind of weird, but it doesn't hurt, so I am thankful for that!
Does the sensation only come in your left knee? Have you had an injury or surgery there? Has anyone close to you had a knee problem?