Unusual Pains on First Try?

Hello, I'm new to menstrual cups and also to this community.
I purchased my first cup today, a size B keeper, since I am a virgin. I have also NEVER used tampons before. I've also never had a pelvic exam before. I'm 20 years old.
It took me two tries to get the cup in, and that had the typical initial pain but i was able to insert and remove it in only a few minutes. The part I'm not sure about happened after it was in. I experienced pretty severe lower back pain (imagine right at the top of your butt, on either side of your spine). And also a very pressing urge to pee. I took it out immediately and will continue doing short insert / remove sessions to see if I can angle it differently or place it higher to make that pain go away. But I haven't read about anyone else having back pain with the cup.
I guess my question is if there may be something going on up there that I should have checked out or talk to a doctor about, or is that typical or the result of some kind of user error? Or maybe I should just get a smaller one, which would be hard b/c they're expensive...

Thanks for any help or advice,
-LN