I had to cull one of my favourite does today because she was very very sick, and it must have come on very suddenly because I culled her at 1pm today, and the last time I checked them was 11pm last night, so only 14 hours between her looking perfectly normal and healthy and fat (She was pregnant) to me finding her, back arched, stomach hollow and fur blown.
She had no resp issues, but she had greenish/Pale brown/Yellow fluid all over her tail and stomach and it really really smelled bad... when I walked in my shed, I thought one of them had died it smelt so bad.
I have no photo's i'm sorry, but I couldn't leave her to suffer for another second after I found her.
The reason I ask whether it could have been Pyometra is because I know it comes on fast, and that pus fills the womb which could explain the smelly liquid.
When I bred rabbits I had a doe that had very similar issues to your mouse. We didn't have to euthanize her, as she was a Flemish Giant (25 pound rabbit) and vets are more capable the larger the animal is, it seems..... but it turned out that her issue was that one of her kits was blocking her birth canal, causing the baby to die and induce an infection. Infections in the uterus occur very, very quickly, especially in small animals who are housed close to their bedding. It's no fault of your own, just a fault of nature ..
Also, pyometra is possible in rats, which leads me to believe that mice are equally likely to suffer from it.
I'm really sorry for your loss. I'm glad you did what was right for your doe and am sure you'll have much better luck in the future!