This looks like a typical hotspot, this can happen due to various reasons such as allergies, local infection or a foreign body in the paw. the first thing you need to do is to buy an e-collar in order to prevent her from licking it, as she is making it worse that way. there is no over the counter specific medication for this condition as the primary cause needs to be treated, you will need to take her to the vet, she will probably need allergy meds and antibiotics.