The red spots on the inside of the mouth are irritation and possibly the beginnings of infection..

My female retic missed her rat once last year and hit the side of the cage. She ended up breaking a couple teeth and her upper lip swelled out like that. My vet gave her antibiotics but the swelling would come and go about once a month or every other feeding for at least 6 months. I guess it just irritated the area and she probably had tooth bits stuck in her gums which could never be found due to the swelling.

When it happens that fast and there are no signs of RI, it was most likely something that caused trauma to the inside of her mouth.

If you grasp her firmly just behind the head and press the stick of a Q-tip or ball point pen against her lips, she should open her mouth. You have to apply pressure. Do not be afraid to make her open it. Just pressing on the lips works. Then hold the Q-tip (horizontally) against the inside back of her mouth, pressing slightly against where her jaws hinge and holding it at a slight angle to urge her to keep her mouth open.

A pen works great for this too as it's a bit thicker. Holding something longer than her jaws are wide, against the back of her mouth while it's open, will cause her to keep her mouth open for treatment.

Don't feed her for a couple weeks to lessen irritation. Id flush it with saline and try the Listerine to kill bacteria. Then let the vet know how she is.

Increasing the humidity will help too. This keeps and mucous from building up in the case of an infection.

If it gets worse then she needs vet treatment. If it gets better then it should heal on it's own with a little tlc.