Welcome to HCM... So sorry to hear about your problem and I will try my level best to resolve it to the best of my knowledge and experience.. As per your complain if seems that you have Submandibular lymphadenopathy leading to swollen lymph nodes under the jaw.. It can occur due to causes like infection in head and neck region while can also be due to tumor of lymph nodes known as Lymphoma.. I would suggest you to consult a General Physician or General Surgeon and get evaluated and a thorough clinical evaluation and investigations like Biopsy of lymph nodes can help in diagnosis and treatment can be done accordingly.. Aa of now you can start doing alternate warm and cool compresses over the lump.. Once the cause is confirmed treatment can be initiated like antibiotics in case of infection while chemotherapy and radiotherapy for Lymphoma... Hope this information helps.. Regards..