It's not that high up, and every woman is different.

It often hits the cervix because of it's placement. The cervix is very sensitive. The pressure and dryness of the tampon can cause pain. Pain can cause dizziness.

edit: actually it doesn't even need to hit the cervix to be painful, just an object being in a place that isn't used objects being there can be enough.

I believe they're referring to this line at the end.

>Later at the end of the school day they stopped me and asked me something casually as if nothing happened, I felt awkward and didn’t hear them so I just said no before going home.