My JRT Grew a lump in her neck

Hi
My name is Desmond and i'm the proud owner of a 8month old JRT, but she grew a lump in her neck that have me worried. She eats well and pretty much continues as nothing is wrong, she doesn't respond negatively when I touch it so it doesn't seem to be painful. She had one a couple weeks go just a bit lower down the neck and it disappeared a couple of days later and now seems to be the same things just higher up. It starts out small and grows to about the size of a golf ball. I have no idea what it is and are getting worried. Any advice will be appreciated.

Thank You