fingers crossed for you that it is just fecal or a strangely large urate stone. That can happen at times.
What I hope that it's not is a fast growing tumor of some sort. They're rare, but can absolutely cause lethargy, blockages and weight loss. But again... rare. And also this would be an abnormally fast growth.
Sepsis is a possibility if it is some sort of large blockage or bad egg, particularly if it had also caused damages to the inner tissues. I think personally I would have liked for the vet to know for a fact if this was a growth vs. urate vs egg issue before medications. It will suck if she was just going into a shed cycle, but as far as for tomorrow it will at least be useful in preventing any potential infections after the treatment. She'll need to be on the probiotics still because of it.
Any type of intestinal blockage though is definitely something best treated as soon as possible.