Should I be concerned about 15 y/o implants becoming asymmetrical after new muscle growth? (photos)

In general, today's implants are smooth and round due to the concern for textured breast implants (see BIA-ALCL). These smooth implants do have a tendency to migrate over time, and excessive physical activity can make the migration worse. It sounds like your surgeon also did some differential pocket work (appropriately so if they had appeared symmetric) which can also cause asymmetric migration. These are various styles of mesh that can now assist in implant migration, so I would speak with your original surgeon regarding a possible revision with mesh.

Dear jdeebreis,
I recommend my patients to replace implants every 10 years in order to reduce the chance of complications. I understand your concern. However, without a proper assessment, it would be difficult to determine what is wrong. It is best that you visit your plastic surgeon for further assessment or ask for a second opinion. Only after a thorough examination, you can get proper recommendations and advice.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Your photos are not very clear with your arms positioned where they are. You may have torn the capsule some and cause the implants to shift. Muscle building will not affect your implants. Go visit with a plastic surgeon. You may also need a mammogram to check your implants if you have gel implants. You do have some bottoming out with your implants which lets them sit lower than the original position. I would suggest going smaller if you do something with them.

As you have had changes in your weight as well as activity level, some differences in the breasts may be noted. However whenever there are changes to your breasts it's always best to visit with your surgeon for an exam to rule out any concerns. Shifts in position and pain could be related to capsular contracture. I've included a video I hope you find helpful.