Mammary tumors are what your girls most likely have, given their location.
Girls are very prone to getting non-cancerous tumors, and from where you say they are, it sounds like the start of them. They often start showing up at around one year of age or more.
Like I said though, they are typically non-cancerous, and can be removed, so long as you find a vet who knows what he or she is doing! If your regular vet says they are indeed tumors, I would let your fingers do the walking, get out the phonebook, try calling some other vets in your area, and ask if any of them can perform surgery on rats.
Mammary tumors generally come back, but removing them once can increase lifespan quite a bit.
