Same here with Winnie. Seems to be seasonal reaction to grass. For whatever reason it's usually only one paw she works on. They get red, really red, and the licking makes it worse. I've tried 3 things : Benadryl. "Dermapaw" and wrapping her foot in tiny white baby socks (you can get a bagful at Wal-Mart). I use strong surgical tape to (usually) keep it on. Lots of praise and treats to keep the Dermapaw and covering a positive thing. It works but the problem is cyclical.
It could be external allergies. If you are not in an area where outside allergies are present at this time it could be say a new scatter rug, something you clean the floor with? And why one paw I don't know.
I am going to suggest that maybe it be anxiety. Is she left alone a lot? Something else new in the house...another pet, baby, anything that she sees constantly? Years ago we had a rottie/shep mix. I wasn't working when we got him and then several months later I got a full time job. He just did not deal well with that and licked his front paw till it was raw.
Cassie licks her right front leg, especially when she's blowing her coat and the next season's fur is coming in. She had a little hot spot there when she came from the pound. It'll heal up, the fur will pretty well grow back in, and then in another couple of months she's at it again. The vet gave me some hot spot stuff that seems to help temporarily. At first I thot it must be hurting her, but now I don't think so. It's either a habit or an allergy.
Interestingly, now that the puppy is here, she's quit slurping herself. It may be simple boredom. The pup keeps her so busy she doesn't have time to lick!