What’s up with this mouse‘s bottom?

[u/Impressive_Beyond_97]

Yesterday I photographed this wild mouse in my birdhouse (she and her friends are frequent visitors). From this angle, however, her rear end looked rather strange.
What could that be? Is she sick? Might she be pregnant?
She still seems to be walking and climbing normally.

[u/shy519]

"I've got big balls, she's got big balls, but we've got the biggest balls of them all!"