The weight of breast implants causes changes in the tissue envelope over time. Often this results in thinning which enables the implants to move with position changes. Without more information specific to your situation it is not possible to offer specific advice.
There are many reasons this could have happened. Some of the most common include breast implant rupture, capsular contracture, and natural sagging of the breasts that occur over time. I would recommend you see a plastic surgeon since it is very difficult to determine the cause without a physical examination. If the physician is concerned you could have a ruptured implant, they may send you for an MRI to confirm this diagnosis.
There are many possible causes for the changes, but you will need to see a plastic surgeon for an exam. Here are some, in no particular order: body changes (weight loss, weight gain, hormonal changes), implant leakage or rupture (are they saline, silicone, or combination?), capsular contracture. At 12 years, implant replacement should be a consideration regardless.
You could have a deflation of your implant which would certainly effect the shape. You could have capsular contracture or a tear in a capsule which could alter the shape and cause odd angles. I would see you surgeon because the last 12 years have seen new implants which might be better for you than the ones you currently have.