14 months after tummy tuck with muscle repair, bulge in lower abdomen when I sit. Is an upper body lift my only option?

Thank you for your question, and congratulations on your excellent tummy tuck results! Your standing pictures show a very tight and well-healed abdominal wall, which is a testament to your surgeon's skill and your body's ability to heal.
The slight overhang when sitting is due to the relaxation of the skin and tissues, which is a normal and necessary adaptation of your body to different positions. If your abdominal skin was too tight when sitting, you would have discomfort and difficulty standing upright.
If this minor overhang continues to concern you, there are options to consider. An upper body lift, also known as a reverse tummy tuck, might help by pulling the lower abdominal skin upwards. This would be done through an incision underneath the breast. Another option could be a vertical abdominoplasty, where an incision is made from your sternum to your belly button and then down to the pubic region. This procedure involves removing skin vertically, which can also help to reduce the appearance of any overhang.
However, these procedures come with their own risks and benefits and should be thoroughly discussed with your surgeon before making a decision.
Sincerely,
Dr. Katzen

From your photos it does not appear that an upper body lift will resolve your concerns. It appears you have visceral/internal fat that is influencing your current look. This is the fat located behind the muscle wall and can only be addressed with weight loss. I encourage you to address that first and see if your concerns improve.

Everything must push forward when you sit. You appear to have gotten a nice result. Try losing some weight so you have less internal fat.