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Sometimes the field is clouded because of things that we cannot see inside the cat.
I am glad to hear that he did the exploratory surgery. This is often the only way to find out what the problem is. This is often because of ovarian or uterine remnants that are left behind. It can also be because of a uterine stump.I am hopeful that she did not get pregnant when she was in heat!
I would contact the group from which the cat was adopted and let them know about the situation. They might be able to shed more light on what's going on.
Let me know if you want to discuss this more.