OK, after watching them for fighting etc and having no change (she looks worse, much worse) we took her to the vets (one of the other rats has a tumour and is being assessed for surgery). They said that they aren't fighting wounds. She must have been scratching and cut the skin, and then got an infection, which has caused blisters to form under the skin and then burst. We have antibiotics to kill off the bacteria, so hopefully we will see a reaction soon. Does anyone have any experience with this?