Chloe got a purple bruise near her incision site after her spay that kind of looked like that, it showed up the next day though, the vet said it was fine that some bruise more easily then others.
However, since this showed up so late, I would certainly have that checked out because you need to rule out any kind of infection, especially with still having stitches in. I have never seen a vet put dissolveable stitches on the outside like that though
Chloe had a HUGE incision because she had a biopsy at the same time as spay and her incision was almost completely healed at 2 weeks and you could barely see it by 3 weeks.