My thought is a nail that broke, which is now healed so you cannot readily find it, or bloody urine, so it would not leave marks on your rabbit since it exited smoothly.
Is it possible to send photos to your vet for consultation?
I am also curious on what has been attributed as the cause or reasons your rabbit has been considered a bit more frail. I do not know your vet, but sometimes people take their rabbits to inexperienced rabbit vets, and it can be the vet’s inexperience which cause some of the during and post-treatment complications.
Can you share how old your rabbit is and how long ago she was spayed?
Also, since you mention another rabbit, are the rabbits barred from interacting from each other? Are they bonded? The blood may be from a fight, if she has no markings and they got into a fight through cage bars. Some background on their history would be helpful.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.