Cute pics but we need to see the lump under her armpit. It sounds like a very early mammary tumour...how big is it?
Normally mammary tumours grow slowly but don't bother the rat until it is very large and starts to suck the resources/nutrients from the rats blood stream. Then your rat loses weigh or the tumour affects her mobility. Its best to remove them when small but even bigger ones can be removed later. Malignancies on he other hand have their own nasty agenda and some can grow very quickly and are difficult to remove (which is why we need more description and pics)...Is the tumour fleshy and softish, or hard and firm? Can you get your fingers most of the way around it or is it firmly attached?
Normally mammary tumours grow slowly but don't bother the rat until it is very large and starts to suck the resources/nutrients from the rats blood stream. Then your rat loses weigh or the tumour affects her mobility. Its best to remove them when small but even bigger ones can be removed later. Malignancies on he other hand have their own nasty agenda and some can grow very quickly and are difficult to remove (which is why we need more description and pics)...Is the tumour fleshy and softish, or hard and firm? Can you get your fingers most of the way around it or is it firmly attached?