There are many common menstrual disc mistakes and solutions. It's the most comfortable and easiest to try. This is also my guess, and i had a similar problem to the op with the disposable softdisc (which has the same dimensions as the disposable flex disc)
Menstrual Disc leaking? Learn why | The Fornix | Flex
It would sometimes leak, and it would also feel like it would almost sneak out from behind my pubic bone
The problems all went away when i got a smaller disc.
When you first start using a menstrual disc, it's completely normal to experience a few leaks Don't worry, you're definitely not alone A menstrual disc is different from other types of period protection, so it can take a little time to get used to it The main causes of leaks are usually
If your menstrual disc is leaking, you are not alone It’s a common part of the learning curve for any reusable disc In a systematic review, up to 45% of women found reusable cups difficult to use initially Issues with menstrual disc leaks are almost always fixable, and research shows that once they get the hang of it, 73% of users want to continue
No more bloating, cramping, changing or leaks
Flex® cup is the #1 menstrual cup for beginners If you’re going to try a menstrual cup, try this one