Most likely it is a tumor, a cyst, a growth. Please do not run away screaming in tears because of those words. The only thing that you need to 'worry' about is whether it is benign or malignant. If it is benign that means it will not poise a threat. If malignant then it is a form of cancer that does poise a threat. Most often they are benign - meaning that it can easily be removed (if necessary), or left alone. If left alone the only thing you need to do is keep and eye on it, making certain that it is removed if it becomes to big. Our 11 year old Lab/Rott mix gets growths/tumors/cysts all the time - some dogs just get them - we keep an eye on them and wait it out until they or it is large enough to be removed. We wait because surgery is far harder on the dog and more risky than letting a growth grow. The Vet will take a look, most likely pull out a needle to get some of the material inside of the growth - that is a biopsy. S/he most likely can take it in the back and look through a microscope and most likely can tell if the cells are cancerous or not. However s/he will NOT tell you that it is 100% either way - not if they are worth their salt, they may say something like 'It looks benign, but I want to send it to the lab'. Do not panic, a lab will be able to spend a lot more time getting acquainted with the tissue sample, they will run several chemical tests as well as run a few samples through a microscope. This will determine exactly what kind of cells you are dealing with. Again most likely it is benign. To put this in perspective, if your dog was a human being she would be around 73. Although 7 years to a human year is used, the accurate method is 10.5 dog years per human year for the first 2 years, then 4 dog years per human year for each year after. Dogs have an initial growth spurt until they reach adulthood, then things slow down. Humans of 73 will most likely confess that they get strange lumps and bumps and skin marks which are just old age taking its toll. Dogs get the same thing as well.