Some further information would be required to comment on the condition and treatment. Usually reduced mouth opening can be due to multiple reasons depending upon which factor is present in your case treatment has to be suggested. If there is habit of chewing or smoking any form of tobacco or Supari a condition called osfm develops which causes fibrosis and difficulty in opening mouth as well as burning sensation on having spicy food. This condition is pre cancerous and can lead to cancer or could already have turned into cancer. A visit to the dentist is a must and immediate stoppage of the habit. An impacted third molar which is trapped inside bone and causes swelling on gums in the back could also be a reason for reduced mouth opening. An injury to jaw causing it's fracture or spasm of jaw muscles are other reasons for reduced mouth opening.

Hello Visit a dentist soon and get it checked. Also mention smoking or Drinking alcohol if you do. Because people with this habit are prone to get such conditions. For more information contact me personally. Wish you good health and happiness Keep me updated with the improvements Regards