I'm afraid that your best course of action is to try to scrape up the funds and at least have that new lump tested. The vomiting could be related...or not... Only a vet would be able to tell you for sure. If the lump turns out to be a benign fatty tumor, then the vomiting is due to something else...but after two months of vomiting, it's time to get it checked out.
The initial checks shouldn't be too expensive--they might be able to do a simple needle aspiration of the lump to determine if it's cancerous or benign, and a blood panel might give some good indications of why she's having so much trouble keeping down her food. Once you have some idea of what's going on with her, then you'll be in a better position to figure out a course of treatment.
She sounds like a lovely girl who was lucky enough to find a very loving home! I hope her problems turn out to be relatively minor and easy to fix!
