Something happened though. Maybe I neglected it far too long in by bathroom shelf after my last period.
In the pictures you can see a mami small. When my last period ended in the middle of november I washed it with lukewarm water and mild soap and put it down to dry on a piece of clean toilet paper in by bathroom shelf. It was exposed to air and touched nothing else. I then completely forgot about it and took it out three weeks ago, so a little more than one month had passed. I noticed this evenly shaped white residue, like someone had poured a bit if milk in the bottom and then let it dry out, and my first though was "bacteria". I scraped it a little with my finger and it came off easily, but not completely.
I boiled the cup for three minutes and let it dry. To my surprise the residue wasn't completely gone.
My period is closing in on me, so I decided to ask for clues and advice here. What could this be? The tap water where I live is very hard and known to create extremely much lime residue on dishes and bathroom floors. I'm quite worried and will not put this inside of me until I can be sure it's safe.
Here's a link to pics in case they act up (I posted from my phone and on the run):