6 months post-op, bump on incision - any ideas what this could be? (Photo)

Just based on your photos, you appear to be making a hypertrophic scar. Yes, you had internal stitches. I would suggest going back to your surgeon and discussing this scar.

Hi and welcome to our forum!
From your photos, I note a healing scar with residual redness (it may take a year or more to fade). There appears to be a thicker portion at the junction of the central and lateral aspect of the scar. The resolution of the photograph limits an accurate evaluation (a red blur).
Irritation from an ill fitted bra may be a possibility. Sudden development of a keloid 6 months after surgery is an unlikely possibility. A deep suture infection may appear in this manner (and may occur months after surgery).
As this entity is only of one day's duration, give it another day or two. If it doesn't abate or if it worsens, revisit your plastic surgeon for evaluation and treatment.
Best wishes...