My mouse has a new, moveable bump on her neck. What should I do?

Thanks for your question about Tiny, and thank you for the photos - they are very helpful. Based on the location on the mass, and how fast it has come on, I would be concerned that it could be an infection, or potentially a tumor. I would recommend taking Tiny into a veterinarian. They can perform an examination, and can put a needle into the mass, remove some cells, and put the cells under the microscope for evaluation. Depending on what they find, they may have medications that they can prescribe (for example, antibiotics if it is an infection). Please don’t give any human medications to Tiny, as many of them are harmful to our furry friends.
I hope that gives you a place to start, and that Tiny feels better soon.