Sudden Weight Gain and Poopy Butt

[JesssPets]

Hey guys,
I'm really worried about one of my rabbits and was wondering if anyone had any ideas about what might be wrong with her, I am planning on making a vet appointment for tomorrow but just wanted to know if others had experienced this.
She's a two and a half year old flemish giant mix and normally weighs around 4kg but has recently reached around 5kg within the last couple of weeks. In terms of diet, I have recently halved the amount of pellets the rabbits are getting out of concern for her weight gain, and for exercise they are now getting around 5 hours a day free roam in the back yard.
When I checked the rabbits this morning I noticed that she had a poopy butt which I have never noticed before so I cleaned her up and sent her on her way to spend some time outside with the others.
Any ideas what could be wrong with her?
Thanks!

 

[Mariam+Theo]

Is she eating normal, playing normal, and acting normal? Does her stomach seem bloated? What does her diet look like? Is she sitting hunched like the rabbit in the picture?

 

[JesssPets]

Is she eating normal, playing normal, and acting normal? Does her stomach seem bloated? What does her diet look like? Is she sitting hunched like the rabbit in the picture?

She's eating normally but has been spending more time hiding out in their shed and pen rather than exploring the garden. We recently got a dog and so I put that down to stress from the dog but evidently it may be something else. When out in the garden she's still content to graze with the others but does spend more time alone than usual.
Her stomach does feel bloated and visually she is rounder than usual.
The rabbits are getting less lettuce than they normally would but they're still getting two servings of leafy greens and a "fun" add-in such as broccoli, celery or capsicum each day.
I haven't seen her sitting hunched, she still sits normally and stretches out as she typically would.

 

[JesssPets]

Update:
Her stomach wasn't as firm to the touch this morning as it was yesterday but she did have a poopy butt again. She's off to the vet this afternoon so hopefully they can figure out whats going on.