I know it sounds unlikely, but I think there is some evidence in the chapter to support this idea:
First of all, the first pages of the chapter are filled with phrases such as: "Some mornings...", "Sometimes...", "Those were the nights...", "One day...", and so on.
Then there are sentences such as these: "They rode every day (...), avoiding towns and villages (...)", or "Aside from that one time, they hardly talked at all. Whole days passed when neither of them said anything.",
Here I was already wondering if the events in the chapter all happen a chronological order and if these passages describe the time before the discovery of Cat's body, or if this is a just a general description of their whole journey (because some days plus some nights seem to be more than 3 days I think).
Then they reach the foothills of the Mountains of the Moon where Arya has the dream of Cat's corpse. Considering that she needs six more days to reach Saltpans after leaving the Hound, it seems somehow unlikely that the ride from the twins to the mountains just took 3 days... in particular when they had to avoid main roads and villages.
Also, this is the time when Arya sees the stars and the moon for the first time again. This is a pretty important detail, since it has been raining most of the time in Cat's and Arya's last chapters. However, as she goes to sleep that night and enters Nymeria's mind, the sky that she sees is described as "grey and thick with clouds", and the river as "green and full of floating things". This is strange for two reasons I think: First, there is again a change in weather (like, why did he mention the first in the first place?). Second, if the scene takes place at night, then the sky should be black anyway, just like the river (Fevre Dream contained many descriptions of rivers by night, which is why this stood out to me in the first place). The same goes of course for the yellow and pinks cloaks of the riders that Nymeria sees afterwards. Jojen even tells us directly that all cloaks look black by night. So in sum it sounds as if this is happening during the day.
Considering all that, it seems as if Arya's and Nymeria's timelines don't run in parallel, but that several more days have passed for Arya between the twins and the dream.
Of course, all these things can be explained in other ways that do not involve time travel, but I thought it would fit nicely to Leaf's warning not to call someone back from death...
I didn't consider that actually, I would have to check again where the pack was last seen and if Thoros specifies where exactly they found Cat...
