I thought her breathing sounded a bit conjested in the first part of the video.
Is it?
If she is off, she is likely ill ....
This is a good time to take her to the vet, have her put on antibiotics such as baytril, and lasix, etc if she needs them and make sure her belly is ok, see if she needs to be treated for an inner ear infection, etc
The weight gain concerns me too.
Many vets will be going on holidays for a week or two and if she gets really ill, emergency vets may be all that will be available until Jan. Emergency vets are very very expensive.
This may not be the case where you live ... it is here.
If you think there is something wrong vets ging on holiday would be one reason for making an appointment asap instead of waiting a bit longer.
You are right, rats get sick and die fast so they can't wait very long when they are ill.