Is it soft and squishy or firm? If soft like a sac of liquid and a bit warm compared to her body temp, it might be an abcess. If firm and you can move it around a bit, it's more likely a tumor.

Might as well euthanize if you dont have money to treat it. CO2 her...

Question... are you housing them in a plastic drawer set up using bowls for water? .... How's the ventilation it that?