It’s good that she has the option to leave the room – always best when they can choose when and if they want to explore and/or socialize. The heavy breathing is a bit worrisome though. A vet check is always a good idea with a new bun, but a vet visit is of course always stressful too, so it’s not so straightforward to recommend it with a bun that seems stressed out as it is. Breathing issues can have to do with the heart. I’m assuming there’s no sign of runny nose/eyes or any weird sound when she breathes? Snuffles (bunny cold, cause by bacteria) is common in rabbitries.
Does she have a safe hidey place where she can get away from practically all stimuli? Normally you’d use a cardboard box with two or more “door holes” but a Flemish is of course a bit too big for most boxes. Maybe you could rig sth with fleece blankets and a small table?
It’s very good she’s eating, drinking, pooping and grooming Cable.