What is happening to my cheeks and jaw? Is this masseter muscle hypertrophy or are my cheeks sagging?

I recommend getting a formal consultation. I would focus on preventing sun damage by protecting your skin with sunscreen, hats and clothing. Have your dermatologist recommend a medical-grade skin care regimen with retinol and hyaluronic acid. Get regular chemical peels. If you do start to see some jowling, the first surgical procedure I’d recommend is a mini lift, with or without a neck lift, depending on your needs. Filler can be used to improve nasolabial folds. I hope this helps.

It would be very hard to give you good reason for sagging based on your photos; all the above may be possible causes. Evaluation and consultation with a board certified plastic surgeon would give you a chance to address your concerns, goals and possible options. Good luck.