Yes, agrees the new vet at the clinic, looks like a mammary tumor. Given the respiratory ick (and her age, and the fact that she's a rat), she's a really lousy anesthetic risk, so we're not planning on surgery. If it's growing slowly, something else will probably kill her before it starts to bother her; if it's growing quickly we can try surgery at the point at which not waking up from the anesthetic is better than a continued lumpy existence.
Her kidneys are doing great, though, so we have permission to dose her with Metacam (an anti-inflammatory / painkiller) up to once a day as needed. I offered her a dose on a bit of bread last night, and at first she didn't seem interested. Once I told chinders that I didn't think she was going to settle down enough to eat, so maybe we should try again in the morning before she got regular food, Hera decided that bread sounded like a great idea and ate most of the dosed part (but not the crust -- picky thing).
So then I told her, "You're a very sick rat and you're going to die soon."
Reverse psychology. Worth a try.
 We've had her for two years this month, and she was an adult then. We guessed at about a year old when we got her, so the vet has 2 years 10 months as the current estimated age in her chart (she started looking younger when we'd had her for a bit -- must not have been fed real well in her previous life), but she could in theory be anywhere from 2 years 5 months up. So she's certainly old, but we do not know whether she is Really Impressively Old or not.