I Had Sub Q Injected Onto my Cheekbones and 3 Months Later Swelling for 5 Days - Why?

Usually we don't see swelling intermittently from Restylane or Juvederm as the molecules can only hold so much of your water and it holds it to the max until the Restylane or Juvederm is dissolved completely.  Deeper fillers and combination fillers may create different reactions, even granulomas or immune reactions, and there is a possiblity of biofilms covering the filler molecules which can be created by bacterial seeding either during the injection of not done in an aseptic manner, or if bacteremia occurs during dental or medical procedures. If the enlargement is accompanied by pain or redness then you should have it checked out by your doctor as antibiotics might then be indicated.

A common side effect due to the water retention of any HA. Contact the injecting doctor so he can observe you closer. From MIAMI Dr. darryl j. Blinski

All HA fillers are associated with water to make a gel in the tissue.  This is how they work.  Some individuals find that the amount of water in these gels can fluctuate.  The menstrual cycle, salty meals, and even an intense workout may affect this balance.  You will need to decide for yourself if you can cope with the product.  It may not be right for your particular face.  If the swelling becomes too much, the filler can be adjusted with enzyme.