She peed and there was blood on the tissue paper and a little bit on the toilet seat. Not a lot at all. She is very healthy physically and mentally so I ‘m not worried that she has been traumatized, but it is unusual. The only thing I can think of is that her hymen has broken from physical activity, she does gymnastics, dance and yoga.
As it turns out she had a microscopic tear that caused the blood. The likely cause was toilet paper or her fingernails.

Thanks for your responses!