Go get that lump checked out. Please don't be too scared to, because it may or may not be cancer, and even if it is, the quicker it comes off -the better.
My dog has 4 breast lumps. She has not been spayed either. I keep a very close eye on those lumps, feel them carefully every week or so for any changes.
She has had all her lumps checked out by the vet, and they are benign. So many lumps are, but it is wise to immediately get them checked.
Self-diagnosis is not a good idea, but I will just tell you....if the lump is very round, and extremely movable it most likely is not cancerous. However, it IS possible for certain types of lumps to turn cancerous after a period of time. So the thing is, even with benign ones, to keep a very watchful eye on them, even if the vet says they do not necessarily need surgery.
It's impossible to tell whether or not a lump on your dog is cancerous without having it examined, and perhaps even biopsied, by a qualified veterinarian.
The fact that your dog is 7 years and has never been spayed does greatly increase her chances of developing mammary cancer, though, so my recommendation is to make an appointment ASAP and have her checked out.
Spaying is SO important, not just for the obvious reason of preventing the births of unwanted puppies, but also because research statistics have proven that if you spay a female before her first heat cycle, you almost completely eliminate the chance that she will develop mammary cancer later in life. Even after one or two heat cycles, spaying greatly reduces the chances of mammary cancer. As the number of cycles goes up, so do the chances that the dog will develop mammary cancer or pyometra, which is a life-threatening infection of the uterus.
I am not meaning to scare you, I just need for you to be aware of the facts. Please keep us posted as to how you make out at the vet. Good luck!
Ghilly