That is an extremely ill little mouse. Her ears are down, her fur (including the fur on her head) is puffed up, and she is in the classic 'I'm in a ton of pain' hunched over position.
I also see flat out gasping, nevermind the abdominal breathing with accessory muscle usage (means she is straining and struggling for each breath), plus she's tachypneic (similar to hyperventilation). It's hard to see in the video, but I would bet her eyes are beginning to bulge as well from lack of oxygen. Can you run your finger over her back and feel her backbone? She appears malnourished (due to illness, not starvation as a primary cause).
Does she feel colder and weaker than normal to you?
I had a mouse die recently here who was very sick for a long time and unfortunately looked/acted like that for more than a week before she eventually died. I autopsied her (and let me point out here that I am a mere nursing student, not a doctor or vet!), and her lungs had been compressed by her heart, which had enlarged. Her liver and kidneys were enormous as well. I really have no idea which organ system failed first, but I would guess it was a cardiac issue to begin with. Her lungs didn't even have fluid in them or any discoloration at all.
I am keeping my fingers crossed for you at the vet! It really makes me feel horrible to say this, but your little mousie looks extremely unwell.. and may be at the point of no return (meaning she cannot improve because of multiple organ system failure). It may be best to put her down.
I'm not sure if you'd want to do this, but you might want to ask the vet if he/she would do an autopsy for you. I remember years ago my small animal vet did it for me for free on my guinea pig. She determined he died of liver failure, and I was later able to connect that to the type of bedding I was using - which saved future guinea pigs from a similar fate.
You and your little mousie are in my thoughts! <3