If the vet did not xray her, it is possible that she has internal laying or egg masses inside the abdomen. But it could be cancer as well. I would say that you could watch her for a couple of days, spend more time feeding her, and then decide if she looks to be suffering. If she feels like eating, she may still have some time. I don’t think you need to rush to euthanize her unless she is suffering. You could make an appointment for later in the week, and if she seems to be in pain or miserable, then take her in. I think only you would be able to know if it is time. You may want to have your state vet do a necropsy on her to find out if she had Mareks or not. What state are you in? Here is a list of state poultry vets if you would like to contact them for information:
https://www.metzerfarms.com/PoultryLabs.cfm