Hello,
If this came up fairly quickly, then I would want it checked out. This is not something that needs to be done on emergency, but in the next few days I would get it checked. Basically your vet will take a needle and get a sample of the cells in this mass to look at under the microscope. This is usually enough to either get a diagnosis or to at least see that there are abnormal cells that it needs to be removed and biopsied.
As far as her appetite, unless this is an abscess, it shouldn't be making her not want to eat. If this is a cancer or abscess, it may give her a fever and that could be causing the decreased appetite. You can always take a rectal temperature using a digital thermometer to rule this out. Normal dog temp is 100-102.
Hope this helps, let me know if you have any other questions.
Dog Veterinarian
DVM, Emergency Veterinarian; BS (Physiology) Michigan State Univ