thanks for your query, i have gone through your question...the lump could be because of periodontal abscess
or periapiacal abscess(tooth infection
) or it can be due to periapical cyst
also... if the swelling is hard it can be some bening lesions like peripheral osscifying fibroma
...you need to undergo some radiographic investigations like IOPAR, OPG.. consult a oral physician for complete examination and diagnosis...