May be ur skinny so ur cheeks have gone inside or u have back molars missing upper nd lower so cheeks go inside,if missing teet,get it replaced,or if u due to tobacco usage tan u will have fibrous bands developed on inner lining of ur cheek tissue which causes difficulty in opening mouth wide or burning sensation with masala nd spicy foods nd tobacco chewing some persons causes also malnutrition which results in cheeks going inside,get it checked for right cause

Hello, you need to consult to an Oral surgeon for this problem.