Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns.. As per your complain the two main structures present below jaw line are submandibular group of lymph nodes and submandibular salivary gland which is a major salivary gland.. A swelling under the jaw that is painful can be due to swelling of either the lymph node the salivary gland.. In case if the swelling is small nodular and mobile it can be due to swollen lymph node and is commonly due to infection in head and neck region.. But if the swelling is firm, diffuse and non mobile it will probable be the salivary gland.. It looks like due to acute infection and you should not delay and consult an Oral Physician immediately so that he can do a clinical examination and investigations like x ray or ultrasound of the area to rule out for the probable cause of infection and treatment can be planned accordingly.. As of now you can take Ibuprofen or Naproxen for relieving pain and do cool compresses for reducing infection.. Hope this information helps.. Thanks and regards. Dr. Honey Nandwani Arora.