Weird ordor smell

I wear a menstrual cup, and I went to change it, as I got dumping the blood in the toilet I got a wiff of a unpleasant odor that came from inside my cup. I don't really know how to explain it, it just smelt bad. Even after I got done rinsing it, it still had the smell. I boiled it a few days before my period, and I think the 3rd day of my period. I left it in all day today because my flow is light. Could anyone explain why there was that ordor??