Hi! It might be the start of an infection due to a little scratch or wound that can get pretty nasty.
I would call the vet, because it might get bad fast and over the weekend it's harder to get to see someone. Maybe get some antibiotics in advance and keep an eye out if it gets better of worse over the next 24 hours?
My rat had something similar and it got pretty ugly quick, but he after s few days on meds he recovered completely!