If you are able to gently touch it, you will be able to tell if it is hard or soft. If it is soft, it may be a cyst which could be relatively harmless – your vet may be able to drain it. However, rats and mice are fairly prone to developing tumors – if it is hard to touch, your mouse might have a dangerous tumor. The fact that it has reached this size would suggest the outlook would not look very good for your mouse. However, only the vet will be able to tell you what exactly is wrong with her – so don’t delay.