My knee has been popping out why?

in the last week my knee has popped out of place 3 times. i have no idea why it does this because i havent really done anything that i know of to hurt it. it happens like this: ill be sitting down somewhere and then go to move my leg slightly and my knee popped out, it felt stuck like i couldnt move it but it wasnt very painfull, but then i moved my leg a little and it popped right back in. this has happend 3 times and i have no idea why. its more scary then it is painfull