Thank you for saving her! I think your theory seems reasonable. She could be fat and have too little muscle from sitting in a smallish cage all day and being bred continuously. Or it could be loose skin from having been fat. You should be able to feel the difference, though. You can always feel the spine and hip bones on a bunny because of how they’re anatomically built, but on a fat bun you can’t feel the ribs. There can be pouches of fat on the sides behind the “arms”. On girl buns there is as a rule a hefty dewlap.

The poop weirdness could be due to her being in a new environment. There’s always a certain amount of stress when a bun goes from it’s well known environemnt to another place, even if the new place is a million times better. The bunny gut reacts, the pH changes in the cecum and the gut microbiota gets disturbed. Also if she’s on a new diet and her gut isn’t used to hay/greens, the gut microbiota will need to adapt. It will adapt, but it can take a couple of weeks.

I think the best approach is to ensure a good diet with plenty of grass hay and a good amount of exercise every day. Although for starters she might not be keen on exercising, it can take a long time for a bun that’s been caged 24/7 to understand the concept of exploring and moving around. It’s like they have an invisible cage around them at first, so to speak.

Weekly weighing is good, and you could also take a picture of her poop every day just so you can see how/if it changes.

Wishing you the best of luck with her, it sounds like she’s won the lottery with you =)