It has been found in Montana, but that is still a big jump. But then again the virus is moving very rapidly right now due to spring time (it basically jumped over Wyoming). Since you are inside, it is less likely. There are a lot of things that can cause sudden rabbit death though, unfortunately. But until you know for sure I would treat it as though it was RHD. So you will want to quarantine your other rabbit for at least 14 days, which I think in this context means make sure not to visit any other friends with rabbits until you have some answers.
I would contact your vet and tell them what happened. They may want to send the carcass in for testing, and can at least give you some disposal guidelines. You are right that since she was unspayed, it could have been a reproductive cancer or something similar, but usually I wouldn’t expect a sudden death with no warning signs in that case.
If it was RHD, there isn’t much you can do other than hope your rabbit has some resistance and provide supportive care if you start seeing symptoms (such as fluids, etc.). There are some cases where rabbits survive it.
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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.