First of all, you all rock and I would have never dared to try a menstrual cup if it weren't for hours of reading the posts in this community!
I recently decided to give this whole menstrual cup thing a try and got a small Si-Bell cup (i'm 22, sexually active/never been pregnant, super high cervix to the point where i can never reach it). The instructions in the box recommended that I first try it during the last 2 days of my period. I used the cup for the last 2 days of my last cycle, and it was awesome -- super comfortable, I was able to remove it without going to ER (lol), I couldn't feel the thing day or night, and most importantly -- it didn't leak one bit, not even the "residual slobber."
Today I got my period again, and I was really excited to try the cup during the first few days of my period, so I hopped in the shower, got myself to relax, and put the cup in using the C/U fold as I did during the last cycle and the dry runs. And it didn't work. I checked to make sure the cup was fully open, rotated it, the cup was sitting in there comfortably, but even before I got out of the shower it started to leak. And this wasn't residual slobber, this was like a full on stream (ugh tmi, i'm so sorry). 5-10 minutes after I got out of the shower it was still leaking bad. I took it out, and I couldn't figure out if there was stuff on the outside of the walls of the cup, but there was some INSIDE of the cup. So the cup is doing what it's supposed to do, and my cervix is still above the cup somewhere, but why is it leaking?
Update: it appears the problem has been solved! After reading all of the wonderful comments, I tried inserting it over the toilet instead of in the shower. I let the cup open pretty much as soon as the rim was past my pubic bone, then relaxed and firmly wiggled it up into position (previously I would put the cup in position before popping it open). I also squeezed the base of the cup once it was open and felt some of the air leave through the suction holes up by the rim. Finally, I gave it a 180 degree twist one way and then the other way to ensure I have a good seal and as of now, there are absolutely no leaks. Thank you to everyone who left a comment with a tip! My periods are a nightmare and this is so liberating! I will never have to cancel plans due to my period anymore! :)