Same brand doesn't necessarily mean it's ok. If it's a higher priced pet store brand like oxbow or sherwood, then I would presume the feed is ok as there is better quality control with those. But if you feed a bulk farm store feed, cheaper ingredients are used and quality control isn't as strict. So more of a chance that even though the same brand, there can be a change of quality with different feed batches. So if you use a bulk feed like this and it's a different batch number than the last bag, there is a chance it could be a problem with the feed itself causing her to not want to eat it as much. And the problem could even possibly be mycotoxins or mold, which is bad. Or it could just be one of the ingredients is off in some way, making this batch of pellets less palatable.
The more hay that is eaten will usually make the fecal poop look lighter in color and bigger, but that will depend on the coarseness of the hay and protein content. A leafier green hay with more protein generally means slightly darker and smaller poop. A coarser hay with more indigestible fiber means lighter colored and bigger poop. But if she is only eating slightly less pellets, then the poop won't change that much. If she is eating a lot more hay, no poop change could just be because the hay is leafy and isn't adding a bunch more indigestible fiber.