Am I using this thing wrong?

I'm from south africa, we already dont have much of a variety of well known menstrual cups, and the big brands cost us a lot of money ( like way to much money) so expensive that I could buy 20 2 litre milk bottles 😂 anyway, I bought a menstrual cup before my previous cycle, I tried to use it but couldn't get it to pop open ( insertion and removal went great) now I started my cycle again this month and I'm having the same problem, I just cant get it to pop open and I feel the cup when I sit or when I close my legs tightly. Do I need a smaller cup or maybe a cup with a smaller diameter?