Why, while i,m walking, my knee pops out of place and pops back in and then hurts like heck.?

while i,m walking along pain free minding my own buisness, all of a sudden my left knee will disengage itself and pop back into place
and then it,ll be sore where i,ll have to rub it like crazy. liniment takes the soreness away. i can tell when we are having a cold wet winter by my bones.
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get it fixed now ,,it will only get worse..the kid next door could pop he arm in an out..cool trick till it poped out when he didnt want it to.