Why does my Fancy Mouse have a large bottom?

She's actually fairly small, but for a couple months, her bottom has been swollen. My brother's 2 mice have died recently and now, she looks sick. I have a healthier mouse as well, who is alot older. What could be wrong? I do not own a male mouse.
Here is my reply to vet.

The mouse may have a prolapsed rectum. It's a shame she won't be able to go to a vet.

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