Hi With your query you did she had cheek bite? and any habits like smoking or pan chewing etc.. or wide opening of mouth while yawning are important to rule whether she had any TMJ problem or not? and to rule out changes in the buccal mucosa. Secondly, it us important to know the status of tooth whether is there any decay or swelling inthe vestibular area or fillings and presence of any wisdom tooth where in an impacted teeth with presence of infected pericoronal flap covering the tooth with limited mouth opening could also be the reason so, it is important for us to know details of which you haven t mentioned any thing about it. Are there any any ulcerations or white patches are seen then it could be a oppurtunistic infection. I would suggest you visit near by dentist since your history is not clear.hope this is useful.