Why did my hot chocolate stink?
Deliberately burn some chocolate so you have a reference to compare the smell with. Burnt chocolate does smell bad.

If you were heating the chocolate directly over a heat source -- and not e.g. using a water bath (which protects the mixture from overheating) -- then chances are that you did burn some chocolate/condensed milk, and then stirred the burnt material into the mix. Letting the mixture sit may have allowed the burned solids to accumulate at the bottom of the pan, making the smell more obvious when poured out.