My black lab has a round sore/wound on her side of her front paw?

How could she get this? what could it Be? she constitutionally is licking it. it to me looks like a blister that popped but its on the side of her foot not bottom. she barley goes out side if only to go potty and run for a minute but spends 95 percent inside all day. I cleaned it and put cream on it and a gauze and then a sock to keep her from licking it. what should I do? let her lick it? how the heck would she get this. when I touch it I dont think it hurts her because she just licks my hand when I touch it.