I have been using my Mooncup for 2.5 years now, regularly boiling it in water for about 5 minutes before keeping it at the end of each flow. A few months ago, it started to have these rough spots at the base near the stem. Now the spots kinda spread to the whole cup (picture in spoiler as reference). It's only on the outside of the cup, the inside is perfectly smooth like how I first got it. I was searching around the Internet for explanations or solutions but unfortunately couldn't find any. Any ideas? Should I boil it longer or add something into the water to boil it?