It could be the wrong size or just how your body operates. I use a disc and cup and I've noticed that they will fit differently on different days of my cycle like some days it works perfect and other days I can't even get it in right to use it at all. You could try getting a range of sizes or even a different brand bc theyre not all the same. I use different brand cups for different days depending on how my body changes in my cycle
Everyone’s bodies are different and certain birth control methods will work well for some but not for others. I don’t think you should be having the pain you described and your best bet is to speak with your obgyn. Maybe stop using until you can speak with them. Good luck