Shelby does the same thing, and so does my mom's pug. We use the same vet, and they said it's a form of allergies - whether it be to the outside or food, but they couldn't determine from where. I have to give Shelby benadryl when she's doing it really bad. She also gets saliva stains on her feet from chewing on them.
Koda is allergic to the grass, but it's not severe. Whenever he's outside, when he comes in his little belly and feet, or anywhere he has any white, is solid pink and splotchy, but after 30 mins or so it goes away.