Hi, As per your query and presentation of symptoms of reduced mouth opening and jaw pain can be due to most common reason that is Oral Submucous. Fibrosis in which there is formation of thick fibrous bands below the mucosa of cheeks and lips leading to stiffening and difficulty in mouth opening. It is a precancerous condition [although not cancer] and should be taken care of.It is commonly seen in individuals with tobacco and areca nut chewing habit. You need to consult an Oral Physician or a Dentist and get evaluated and he can advise him to apply a steroid ointment in combination with hyluronidase containing ointment to break fibres. Intra lesional injections of steroid and hyluronidase can be advised to be injected after every 3 to 4 days. Anti-oxidant like Lycopene is also advised. Physiotherapy like blowing cheeks, trying to open the mouth under pressure etc can be demonstrated and advise to be done daily. Complete cessation of habit of tobacco if you have chewing is must. Hope this information helps. Let me know if I can assist you further. Thanks and regards, Dr. Honey Nandwani Arora