Bumps on inside of cup?

Has anyone had problems with tiny bumps developing on the inside of a menstrual cup?

I've been using the Keeper for about five years, and within the last six months I've noticed that there are tiny dot-like, raised bumps on the inside bottom of the cup, where the blood rests. I've tried cleaning the cup with an old toothbrush and various solutions of diluted vinegar, baking soda, hydrogen peroxide, and diluted tea tree soap. The bumps seem permanently attached, and I can't even scratch them off with a butterknife.

Are these damage or degradation to the surface of the rubber? Or bacteria becoming embedded? My cup doesn't smell terribly, just a bit of "female" sort of smell...


By the way, this is a great resource! Found the answers to a couple other questions already!