I agree, a dog with a lump should really see a veterinarian. Abscess, cyst, tumor.....all three possibilities should have attention. Dogs get a lot of fatty tumors that are not a problem, I would hope that is what she is getting. Even so, I'd have it checked out. Some lumps are cancer and removing them saves the dogs life. I'd make sure you know what you are dealing with!
What they typically do at our place with fatty tumors is this.
They do a procedure called a fine needle aspirate. It doesn't really hurt the dog, just a tiny stick. They pull our a few cells to look at under a microscope while the dog is awake and you are waiting. This can tell them a lot. If they are just fat cells they feel it is a "lipoma" which are ALL OVER lots of healthy old dogs.
If they think it is a malignant tumor, it would be typical to bring your dog back on a different day for a scheduled surgery to remove the lump. Then they send it to the lab. The doctor can tell you what it is usually when they take it out.
Some lumps are wounds or abscesses, cysts. Each of these would require a different treatment but wouldn't be life threatening unless neglected. They drain or remove cysts and abscesses and your dog gets to go home with you the same day usually.
It kind of depends on what they decide it is and needs to be done. Keep us posted!