It is probably a local skin infection, and not a sign of breast cancer.
In the mean time, seek the advice of your local pharmacist about using antiseptic first aid cream. Try and clean the area and apply antiseptic cream and cover it with a light dressing, if it is oozing or catching on your bra, don't use a plaster. Apply the cream well twice a day, for a few days, up to a week and it should clear up. If it doesn't, or gets more sore, or red and hot, in the meantime, then seek more prompt medical advice, as sometimes, not often, antibiotics are needed.
You could make an appointment with the GP, and then cancel it, if it clears up before the appointment comes.
