Hi. Thanks for the query. As per your complain it looks like an obstructed duct of submandibular salivary gland that is present under the jaw.. It is causing swollen gland under the jaw and along with it the swollen hard lump is appearing to be salivary stone or Sialolith.. Salivary stones causes obstructed duct as when there is obstruction in the duct of the salivary gland due to formation of crystalline stones known as Sialoliths from the minerals present in saliva the duct is blocked.. When saliva flow increases suddenly when we start eating and it cannot flow into the mouth there is backflow of saliva into the gland and it swells.. As the salivation gradually reduced and also some saliva seeps through narrow spaces left in the ducts there is reduced swelling.. You should consult an Oral Physician and get evaluated and he can do a thorough clinical evaluation, although as you are pregnant x rays cannot be taken.. If it is suspected to be due to stone formation you can be advised to suck tart candies as at times the stones can flush out with heavy saliva flow.. Superficial stones can however be pushed manually towards the duct orifice and can be removed.. In your case if seems to be superficial stone and can be removed by pushing it towards the duct opening of the gland.. Hope this helps.. Regards.