Are you sure that it's not just scar tissue that has formed? The site was closed in more than one layer with different types of suture and even a surgical glue for the skin. The absorbable suture that was used internally starts to break down over about 2 weeks but that doesn't mean they are completely dissolved in that time and could take longer which it sounds like in this case. Sometimes there is a reaction to the suture but that usually involves an opening to the incision. But there have been times that a stitch comes to the surface and can be clipped and removed by the vet.
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