It could be a traumatic injury to the soft tissues (muscles, ligaments, tendons), such as: brusing, tears, or partial tears, an overuse injury (sprain or strain), or a fracture.
If the lump is not associated with pain and/or limping than it could be a dermal mass, or a mass of the deeper tissues, and these masses can be benign or malignant tumors.
She is 8 years of age, and in great health, however at 8 years she is in middle age, and tumors do increase with age. If it is just a swelling secondary to an injury than the size of the mass should begin to decrease quickly. If the size of the mass stays the same or increases, your best bet would be to take her to your vet and have the necessary diagnostics performed. These can include, but are not limited to X-Rays if your vet thinks it may be a fracture, or torn ligament, for example; or, fine needle aspirate or biopsy if your vet believes it is a mass.
Let us know how it goes!
Thank you very much for your comments, Dr. Cheng.
I was concerned enough about my dog's lump to get an appt. with my veterinarian this morning. After examining her for a while, the Dr. performed a fine needle aspirate at 4 locations around the mass.
1 of the locations yielded fatty deposits and the other 3 produced blood. His diagnosis was that there was most likely a lipoma at that region of my dog's leg originally, but that there was some unknown trauma that caused a hematoma, which in turn made the lipoma more pronounced than usual. He supported this diagnosis by looking at the contents of the aspirations under a scope and saw nothing abnormal.
I was advised to measure the mass for upwards of 4 weeks and bring her back in if the mass should increase in size or if we see any changes in the color of her skin at the site of the mass.
Thank you very much for your help!