It's possible you have an impacted tooth. Another possibility could be an infection of the submandibular and/or sublingual gland. These are salivary glands located on the floor of the mouth near the area you described. Sometimes the gland can become blocked from the formation of a small hard particle that's composed of chemicals from the saliva. These chemicals can then harden into a solid material and block the gland. The glands then become engorged with saliva and may become swollen. This can be diagnosed with an x-ray and treated with minor surgical excision.
Your orthodontist can can diagnose and treat this. I suggest you make an appointment. Best wishes to you.