Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your Lab.
Fortunately, most of the lumps and bumps that dogs this age develop are benign and are no concern at all. They bother us more than they bother the dogs, in most cases. They rarely cause pain unless they become invasive and affect nerves/muscle.
There are several possible explanations for the lump that you have found, though:
1. Lipomas or fatty tumors can develop anywhere on the body. These are totally benign. They can feel firm if they are underneath muscle. Some of them can become quite large; they tend to be slower growing but in furry dogs, could be easily missed when they are small.
2. Cystic mass....also benign.
3. Enlarged lymph node if located on the back of her leg about midway up the leg.
4. If this is a recent finding, she might have a small abscess although these are usually a little uncomfortable when felt and often have a small obvious wound or puncture site.
5. Cancer, I am sad to say, can develop in a dog this age; it's not only the older ones who might have this problem. It wouldn't be common, but it could happen.
6. Warts can be seen on dogs although they're usually hairless and look very similar to human warts.
Unfortuantely, it's often not possible to determine what a lump or bump might be based on feel or visualization alone. Usually cells need to be aspirated and examined under a microscope or sent off to a pathologist for review.
And, when it comes to treatment options, we are somewhat limited....we either monitor them, freeze them off if small enough, or surgically remove them.
But, since this lump is not bothering your dog, I wouldn't consider this to be an emergency situation. I might encourage you to have it checked out if she's due to be seen by your vet but you don't have to take her to an emergency hospital.
I hope this helps. Deb