I agree with your observation; it appears that your left breast is slowly becoming smaller than the right breast over time.
Because this is a gradual decrease in size and you have saline implants (making a breast implant rupture more noticible than if you had silicone implants) you most likely have a slowly leaking or deflated saline breast implant. Saline implants rupture at about 1% per year; unfortunately you are one of those 1% this year.
Here's what to do next:
1. Return to your plastic surgeon as soon as possible (you will want to have the implant replaced quickly to avoid shrinking of the breast implant pocket)
2. Find the serial number and paperwork associated with your breast implants (the company may give you a replacement implant free of charge since you seem to have a malfunctioning implant).
Jaime Perez, M.D.
Breast Augmentation Specialist in Tampa, Florida
Plastic Surgery Center of Tampa, Florida