You might have a ulcer formed there. It may take some time to resolve depending on the severity but you can try few topical medicated gels and diet modification to ease your discomfort. Avoid extremely hot or spicy food for few days. Having cool beverages and applying ice on the affected area will reduce inflammation and pain. You can also start with a medicated non alcoholic mouthwash to hasten the process of healing. For more detailed evaluation you are advised to contact your dentist.