Hey Lilly. Bless your heart, you're going 60 miles an hour here.
I just saw your question, so I'm late writing. Maybe you've already figured out the period question by now. If not, here's an alternative. When I was 12, I filled the toilet with blood after having a bowel movement. At first I thought I had started my period. I called my mom into the bathroom and showed her the toilet. She didn't know what to say. It didn't happen again. She never said anything about it to me afterwards. (When I grew up, I asked her why she didn't. She told me that it scared her and she didn't want me to be scared, so she said nothing. She admitted that it was a mistake, since at the time I felt like she didn't care.) Evidently, I broke a blood vessel during my bowel movement.
I didn't start my real period until I was 13 1/2. And it was definitely a real period.
If she didn't continue to bleed, this might be what happened to your daughter. Alternatively, with the pain you were talking about, I would say that if it persists, and she doesn't bleed outside of sitting on the toilet, I'd take her to the doctor and get her checked for a UTI.
When she does start her period, try small tampons with her since she's an athlete. Give her vaseline to put on the tip, and have her squat over a mirror so that she can see where the vaginal hole is. You'll have to talk about the insertion angle, and then have her sit on the toilet to try to insert it. The vaseline is oh so important. Tell her how important it is to get it in deep enough past the muscle. She shouldn't be able to feel it when she walks if she has gotten it past the muscle.
I would not try to use sponges or cups with a child her age. Let her use tampons right now. The other things aren't necessary and she has more growing to do.