I've owned my (UK) Mooncup for about a year now, and while I love it most of the time, I've been having a strange problem with it. Occasionally while I'm using it, perhaps once or twice per period, I start developing a dull, pressing ache in the section of my back where my right kidney is, so quite high up. It's in a completely different place to my period cramps. It's usually alleviated by me taking the Mooncup out, but there doesn't seem to be any problem with the cup or anything unusual when I do - the little holes aren't blocked, the suction was fine, it was in the same place in my vagina as normal, etc. There's nothing wrong with my kidneys (I've had kidney x-rays and an ultrasound), it's not a kidney or bladder infection, and it's unlikely to be an STD as the only person I've slept with has been tested. I only ever get this pain with the Mooncup in, never with it out - it might stay for a bit once I've taken the Mooncup out, but it'll only start in the first place if my Mooncup is in.
It must be the Mooncup, but I can't seem to find anyone else with this problem. Googling mooncup and pain gets me lots and lots of results about how people's periods are less painful on the mooncup, and that isn't strictly true for me!