I would guess its her Hymen breaking, which happens easily enough jumping round on furniture and climbing trees etc.
I had a bleed when I was about 9 or 10 and was taken to GP. I think they tested for a UTI which was negative and that was about it. They told me that I might get AF soon, but I didnt get AF for a number of years later.

I agree with the other that you should take her to the GP just to get reasurance and any further advise.