Extremely Fatigued Mouse

Question
Earlier today I put my two female mice in a ball for them to roll around in, my dog pushed it around a little bit, but I stopped her and there seemed to be no damage.

I then placed the ball in a different cage, while I cleaned theirs. I didn't watch them from that point on, for the next 3 hours or so.

When I returned, one of the mice was laying on her stomach, with almost no mobility. The other was fine, and knowing that they both got out of the ball, and a lot of poop was spread across the cage, it seemed like this happened quickly and very recently.

I picked the hurt mouse up, she barely moved, but seemed frightened, with heavy breathing - her right leg also seemed much more limp than her other legs.

She has been laying in the cage for about an hour, with absolutely no movement, still breathing rather heavily. There are no other signs of injury, or noises.

I deeply appreciate the time you spend helping people and their pets, I hope that today you can help me. I'm here to answer any questions you have. Thank you.

Answer
Hi Ryan,

I believe your mouse has been injured (you probably guessed the problem was in her right leg), and if she is still with us at this time, she needs to see a veterinarian immediately.  For injuries there is nothing you can do at home to repair it - it has to be examined by a professional.  In the time between now and when you can take her in (though it should be as fast as possible) keep her away from chills or drafts and make sure food and water are very near to her, so she won't have to crawl to reach them.  Make sure no water could be spilled onto her that might make her cold.  Try not to move her more than necessary, and separate the two mice for the time being.

Rodent balls are best for one animal at a time, and most carry a warning not to use them for more than half an hour.  Imagine if you were in a ball with a friend - it would be very difficult to move the ball in one direction at a time!  Hopefully her injury and condition are temporary or repairable, but unfortunately it's impossible for me to give you any specific advice or prognosis from here.

I wish you the very best of luck, and I hope she's okay!
-Tam