It's directly between her front legs, about the size of a hazelnut, and very soft. I wanted to get a picture for your diagnostic pleasure (ha) but she wasn't being very cooperative.
For your convenient diagnostic reference, some common sources of lumps in rats are: benign mammary tumors, malignant tumors of various sorts, abscesses (infected areas), and seromas (areas filled with fluid). Information about Echo's previous lumps can be found here.
This morning the vet aspirated a sample from the lump to see what it was. (Tumors couldn't really be diagnosed from this, but it would be possible to tell whether it was infected and therefore whether she ought to be on antibiotics.)
If we are lucky, this is a result of the hormones that sparked the previous peculiar growths still sloshing around in her bloodstream, and once they're done sloshing (now that she's been spayed) she should reabsorb the milk and just have a harmless little non-growing lump of mammary tissue like the ones that were removed. If we are not lucky, there's something wrong with her pituitary gland, or another part of the body that could produce prolactin, and she's liable to get quite a few more of these.