When I have observed this in the past my best guess was the opposite of stress. The plant has extra energy and resources to put into vegetative growth and the simplest thing to do is to tack on an extra leaf for additional photosynthetic capability. I think it is most likely something that a healthy plant would do, not a sick one. The only down-side might be that this indicates vegetative rather than reproductive growth, so possibly leaves at the expense of flowers, but ultimately a stronger plant. I suppose slightly less than optimal light could be a factor in that. Also, did you repot in the months just before this, or boost fertilizer a bit especially nitrogen?