I would take her back to the Vet where she was spayed. That could be ringworm and she could have picked it up from the Vet's office. It's highly contagious.
It could also be a reaction to meds, etc that they gave her, causing her to scratch herself until the fur is gone and it bleeds.
Either way, since she didn't have this before her spay, I'd take her back to the same Vet for an explanation.