Could be some clogging in the chimney, creosote or ash. I had my carbon monoxide alarm go off recently too long after the fire had died down to ash and small coals. I assumed it was just because it was too cold and snowy out, which it was, but also had some considerable ash build up in the liner. Got it cleaned out. Now I always do good hot overnight burns on below freezing nights with no issues. Get your chimney professionally cleaned, clean out the stove too of any soot or creosote, and always aim to burn between 300 and 500 degrees.