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Was this a canvas you stretched and primed yourself, or was it pre-primed roll canvas which was mounted on stretcher bars?
Correctly done, a "home-made" canvas is usually more stable than those made up from pre-primed roll stock. As Tom notes, seasonal changes in ambient temps and humidity normally affect linen, and adjustments made in "slack" times may be too much. It's actually the major complaint against using linen, which seems most prone to be affected by climate change, and depending on the locality where you live, and where your paintings may be placed, other fabrics may be a better choice. Polyester is incredibly stable and unaffected by changes in the weather. Hemp is also far less reactive than linen.
As Tom notes, Bill Whitaker's suggestion to insulate stretched canvases from behind will not only ameliorate seasonal sagging, but will greatly extend the life of stretched canvases generally.
Perhaps spraying alcohol on the backside of a sagging canvas would re-shrink it, but I think that would only depend on the water content of the alcohol. Rubbing alcohol has a considerable percentage of water in it. Spraying the back with plain water would be a temporary fix, as the canvas is certain to sag once again when that added water has dried. Using an alum solution for the spray can be effective and "permanent" if sagging is minimal. A tablespoon in a pint of water works well. Alum will considerably toughen the glue size.
Mounting to a panel can be problematic: what kind of panel should be used? What kind of adhesive to use? Mounting to a panel may also obviate the main (only?) advantage of painting on a stretched canvas, i.e., the light weight of larger pieces.
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