Ok I do not agree with either vet entirely. She should be spayed again and it should be done when she comes into heat. It is a remnant of ovary not necessarily uterus, although there is some uterus left to get bleeding.
Not having her spayed she is at a higher chance of breast cancer and can also get an infection in the uteral stump.
Good Luck
Dr Dawn
I really don’t have an answer it could be an infection (bacterial, fungal or viral), cancer or a traumatic injury (like a burn). My advice is to take him to the vet for an accurate diagnosis. If he stops eating or seems painful it is more important to get him to a vet quicker, however, if he is still eating and not painful he doesn’t need to see the vet as fast.