Xiao Yao shot an arrow at Xiang Liu, and Xiang Liu was injured and absorbed Xiao Yao's blood. This arrow had broken the relationship between the two of them, and Xiaoyao even wanted to poison Xiang Liu to death. However, after Xiao Yao drank the poison, Xiang Liu came back to save her. He used his own life to remove the poison from Xiao Yao's body. In the end, Xiao Yao learned that Xiang Liu had died. Before Xiang Liu died, he had stored his love for Xiao Yao in a doll, but Xiao Yao would never know about it. This arrow changed the fate of the two of them, and their relationship was separated from then on.
It could be a rogue vampire who has a vendetta against Bella or the Cullens. This vampire might be out for revenge and attacks Bella, which is how she ends up covered in blood.
One possibility is that it's a misinterpretation. She might be covered in something that looks like blood but is actually something else, like a special potion gone wrong in a magical fanfiction setting. This could lead to all sorts of misunderstandings and adventures as she tries to clear her name or figure out how to reverse the effect.