Why is my Control stick acting weird?

So, I have two (2) GameCube controllers, purchased from GameStop (fresh from box) and made by "@Play". After a a couple of days of use, Both controllers' gray control sticks started acting weird. The controller outputs as if i'm holding the control stick, sometimes lightly, sometimes heavily, in one direction, which I am NOT. Is there a cause for this? Is there a solution for this?