Hi Thanks for the query.. As per your explanation of symptoms and location of the swelling as you told that it is located right under the jaw shows that the lump is due to swelling in the sub mandibular salivary gland which is located just below the lower jaw.. The swelling can be due to obstruction in the ducts of the salivary gland due to formation of crystalline stones in the ducts of the gland and they are formed from the minerals present in the saliva.It leads to back flow of saliva into the gland leading to swelling.. It can also be due to formation of cysts or tumours into the glands. It can also be due to infection in the gland but an infection most commonly causes painful and tender swelling.. If there is already any infection in any part of the mouth, face or any other body part and if the swelling is round and mobile can be mostly a lymph node swelling.. YOU NEED NOT TO GET PANIC AS IT IS MOST COMMONLY A NON CANCEROUS SWELLING.. I would suggest you to consult an Oral pathologist and get evaluated for the exact cause of the swelling and get treated.. Obstruction in the ducts can be cleared by locating and removing the stones either by palpating and pushing the stone towards canal opening and in case of large stones a small incision placed inside mouth and stone is removed.. Infections in gland or lymph nodes can be treated by antibiotics.. Hope this helps.. Regards..