I don't know but it might be something to mention to your vet. They sell ointments for paw pads to protect them. Every dog is different. It must be something she is stepping on is causing a reaction. I would try some sort of balm to protect and sooth:
Pal Dog Soothing Paw Balm, 1.7oz
or
Amazon.com: Musher's Secret Pet Paw Protector... Poor thing has some sort of reaction going on. Maybe protecting the pads could help.