9 months post-op, right unilateral breast soreness/tenderness, what could be the causes?

You may have a trapped sensory nerve, a neuroma, from scar tissue. I would suggest trying local anesthetic injections to see if the pain is relieved. if need, see a pain specialist.

Dear rdd18,
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

Unfortunately, there is no way to know for certain what the ideology of your breast pain is.
Millions of women suffer from breast pain whether they’ve had surgery or not.
Most surgical related pain that is chronic starts from the beginning of surgery.
If you were pain-free and then developed pain, it is most likely not related to the surgical procedure.
Some forms of breast pain don’t have an explanation.
Breast pain is common among women and your best source of information is probably your primary care physician.
unless you developing a capsular contraction it’s probably not anything your plastic surgeon can help you with.
I wish I could be more helpful and optimistic.
You are not alone and this happens from time to time.
Plastic surgeons are not particularly good at diagnosing or treating chronic pain whether someone has had surgery or not.
Best,
Mats Hagstrom, MD