What would cause unilateral breast swelling 20 years after saline breast implants? (photo)

An exam and consultation with a/your plastic surgeon is recommended to evaluate your presentation. A capsule contracture, seroma, or other process may require tests and/or surgery with discussion of your options and expectations. 

Fluid collection as in a seroma requires a check as soon as possible by a qualified ASAPS board certified plastic surgeon. I have patients with saline implants without problems for over 25 years. Age of the implant is not an indication for change. More often body change, weight change is a reason for changing implants. If you had textured Allergan saline implants an ultrasound is possibly indicated. It is possible you have mastitis or capsular contracture. However late seroma, one must be wary of ALCL. Good luck...