I am so sorry, but none of us here can possibly tell you what this lump is.
You need to take her to the vet. He/she will examine it, and your dog, and possibly will take a quick needle-biopsy of it.
The needle biopsy will only take about 5 minutes, can be done with you in the room, and doesn't need a sedative or anesthetic.
Agree with ginger. Although it is not unusual for older dogs to get lumps, like fatty tumors, it is always best to get any suspicious lumps checked by your Vet. The older the dog, there is always the possibility of a Cancer.