I didn't have this problem for the first 5 months using it and the last 2 months toward the end of my cycle there has been a bad smell. I have been cleaning it with dr brommers and other natural soap and I have used vinegar.
I know the smell is not a good sign. When I got the Keeper I read the directions and it just said to wash it occasionaly and give it a good clean when period is over. NO mention of boiling it. Well, I know that is not the case. Will boil it. Hopefully that will help. Otherwise I am loving The Keeper.
No smell here. I have a Diva Cup so it's silicone and the instructions say you can boil it. What's the Keeper made of? I'd check, if it's a different material boiling may not be good. You could Google it too, there are message boards out there specifically for menstrual cup support (which I found very helpful when I was starting!) and I'll bet they would have the info.