Needing some advice here please: Brewed a Bohemian-type Pilsner recently, but some mistakes... Tuesday night brewday. Did yeast starter in warm (not pasteurized) water , but had to leave it to sit overnight (covered) as I was unable to cool wort down rapidly from the boil, had equipment issues and water restrictions. Put fermenter in freezer for 7 hours. Removed and temp was 16 degrees, pitched yeast (Saflager W34/70). FORGOT to aerate!!! Fermentation started a bit later into Wednesday. Airlock CO2 was smelling good through to Thursday. Friday - smelled airlock CO2, smelled slightly off like sulphur, bit worried. Would this off smell be due to the yeast starter having to sit overnight and got infected? Or possibly due to slow cool-down time?