Paw of injured dog, advice?

It a hard bulge and the dog has a limp

Hello - I wonder whether this is an interdigital furuncle?  Its worth saying straight away that if so, then these can be painful and a nusience.  They can be caused by a grass seed, a small mite, a stiffly growing hair or some other small foreign body or allergen sliding into the gap between two toes and driving the patient mad.  Frequently, a small cyst forms and lameness (ie pain preventing the patient from weightbearing) is not uncommon. Bathing the feet in warm water can provide some releif.  Some of these dogs have demodex, which can be treated - this is a good reason to show the lesion to a vet.  Some have chronic allergies which can also be treated.  The Merick veterinary manual advocates long courses of antibiotics in some cases, but this can only ever be done with attention to antibiotic resistance guidelines and prescribed by your vet.  Pain releif may be indicated - again, in case you were tempted, please check with your vet before giving.  It is definitely worth an appointment.