Well, realmswalker, you are not complaining of breathing problems, so I suppose the gas mixture was mainly oxygen. In fact, mixing this two instable components will always give you both oxygen and chlorine, and it depends on conditions like pH, temperature and indedd presence of some impurities that can act as catalysts which will prevaiL. And the heat is produced whenever instable components react to produce more stable ones.
"Mixing some stuff" can be dangerous - if the conditions had been different, the mixture might haven contained mainly chlorine. Even at low concentrations chlorine can make you very sick, at least it would have made you run like hell.