I can't open one side of my mouth properly following fat pad removal. What may be the cause ? (I'm a 22 yo male)

It is not uncommon that patients have difficulty open the mouth in one side or both after buccal fat removal. This is secondary to irritation of the masseter muscle, a chewing muscle that is close to the area where the incision for the buccal fat removal surgery is performed . This difficulty to open the mouth could lasts from few days to one or 2 weeks. In my practice I recommend my patients to do mouth exercises and take anti-inflammatory pain medication. 

You are still very early on in your healing process.  Swelling may cause different feelings in different areas.  Share your concern with your surgeon.