If the denture is causing a 'traumatic' ulcer, then this needs to be adjusted by your dentist ASAP. They can also checked that you do not have a localised osteitis ('dry socket' after any of the extractions which can also give post-operative pain. In the meantime, simply rinsing with warm salt water or Chlorhexidine 0.2% (available from any chemist non-prescription) will help, as will Sqad's suggestion of taking 'painkiller'. There is no indication to use a steroid rinse/pastillles at this stage.