You have not mentioned the area of the spot but after hearing your symptoms i am quite certain that the problem is related to your wisdom tooth which is either errupting or else is tilted in the wrong direction. it happens as food gets lodged in that area and its difficult to access it with a brush resulting in infection. this infection in turn causes discomfort in the surrounding gums and also results in mild swelling in the area. warm saline gargles may help and i would recommend you to visit your dentist
who can give medication as well as can clinically see the problem.