This looks like a small stitch reaction or a tiny pocket of fluid right where the knot came out. That's a common, usually minor finding around six weeks after a C-section. Your OB already started TBact for irritation in that same spot, so this new bump is probably part of the same healing process.
Next Steps
Please show your obstetrician this photo this week, so they can confirm what it is in person. Keep the area clean and dry, and avoid picking at it.
Health Tips
See your doctor sooner if you notice spreading redness, pus, a bad smell, fever, or more pain and swelling.