John, you are quite right. I came to the same conclusion early this morning and took Monty back to the vet as soon as they opened.
The vet said that since we first took him last week, and after a week of antibiotics the lump had doubled in size and he is booked in for tomorrow to have a biopsy performed. The vet ruled out a tooth issue as the lump is in the wrong position and he feels that it is either a cyst or growth.
The dog is 3 years old and until last week had not shown any indication of any health issues.