Update:

My sister just called.

The vet gave Shadow a anti-biotic shot and
a stronger perscription of anti-biotics pills
to take for 14 days. She also recieved an anti-biotic spray
for the paw. Shadow needs to go back to the
vet in 2 weeks. The vet no longer thinks it is
due to an allergy, but doesn't know why unless
he does testing.
I told my sister I think Shadow licks out of boredom
and she needs some attention.

So I think Shadow might be getting some needed attention
and some care for her poor paw.