Hi there @kisinclair,
I am so sorry to hear about poor Eve, she certainly seems to be having a rough time at the moment.
Lumps in animals are always tricky to deal with. This is because it is very difficult to tell what the lump is just by feeling it or by looking at it.
I am sorry to say it is hard to make a suggestion as to what might be wrong with Eve without seeing her and performing a physical exam.
I would like to assure you that your vet is doing the right thing and a biopsy is definitely a good way of finding out more information and will reveal what sort of tissue the lump is made from.
This will then help your vet decide on the most appropriate treatment for Eve.
In the meantime, it is important to make sure Eve is as comfortable as possible.