To find out if this is cancer you may want to have an x-ray done of the area or a biopsy of the tissue.
An x-ray should show you if there is a bone tumor. A biopsy would tell you if there is cancer there or infection.
You might also want to do a blood test to check for tick carried diseases or factors indicating cancer.
Tick diseases can cause swelling of the joints and muscle pain.
I know you said it doesn't seem to hurt her but check to see if the muscle on each hind leg feels the same size. If she has less muscle on the side with the lump that would be a sign she is favoring that leg.
If your vet cannot help you then you might ask for a referral to a veterinary oncologist for more in depth diagnosis and possible treatment plans.
Hope this helps you!
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