Hi.. Can understand your concern.. As per your complain swelling on the underside of the tongue which is not stable on one side and occurring on both sides which subsides indicates that there is allergic reaction to some food product or any medicine or any recreational drug if you are taking which is causing swelling in the salivary gland known as Sub-mandibular salivary gland present just below the lower jaw and swelling in it is apparent below the jaw and in the floor of the mouth.. It can also be due to blocked ducts of salivary glands but blockage on both sides together seems a bit uncertain but cannot be ignored.. The ducts gets blocked due to formation of crystalline stones from the minerals present in saliva and when the there is obstruction in the salivary flow there is back flow of saliva to the gland causing swelling specially during meals as the saliva flow increases during that time.. I would suggest you to consult a dentist and get evaluated for the exact cause and in case if it is an allergic reaction cessation of the drug along with taking antihistamines like allegra can help.. In case of salivary gland duct blockage exploration of the stones and removal by either pushing the stones towards the duct orifice and removal or in case of large stones surgical removal can be done.. Hope this information helps.. If you find the answer helpful please write a positive review and click on find this answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora.