Hi.. Thanks for the query.. Sudden swelling of the jaw and neck with slight tenderness that goes down on its own seems to be due to allergic reaction which can be related to any foods, drug reaction, bug bites etc.. So it is advisable for you to get it clinically examined and if allergic reaction is suspected you can be advised a blood test to rule out the cause for the reaction.. For now if there is swelling you can get it tested and take anti allergics like Benadryl and do cool compresses over the swelling.. You can also take anti inflammatory painkillers like Ibuprofen to reduce pain.. Hope this helps.. Regards.