Potassium Chlor or Sodium Clor. Some wineries are infamous for setting off migraines with these preservatives.
Potassium chloride is a supplement used for treating potassium deficincies in humans.
I think the chemical you're referring to is potassium metabisulfite, also known as meta or S02, i.e., sulfer.
Wineries use different amounts of the stuff in winemaking and there are federally imposed limits on total amounts allowed.
As some people are allergic to sulfites, this may be what causes headaches. At any rate, that's why all wine in the US is labeled with a warning that the product contains sulfies.
The stuff, incidentally, is used as an antiseptic, to preserve color, bouquet, etc and for it's properties in halting fermentation. Sulfites are also produced naturally in the fermentation process.