Is the lump under the skin?
Were they dissolving stitches, or normal stitches that were later taken out? If they are dissolving stitches it's possible that part didn't dissolve properly and is causing some irritation. I had that problem with Fey when she got stitched up after an accident. My vet just took her into the surgical room to make a tiny cut so he could see what was in the lump since it didn't feel right for an abscess. He saw the thread and just cut it and pulled it out. Voila, no anesthetic, no surgery, no antibiotics. And fortunately for me, no charge!
I'd have it checked out. Hopefully it's an easy fix like mine was- Fey's started out like a really small lump but the vet couldn't see her for several days. By that time it was larger and red with irritation. There is a possibility for an abscess, but that should be less likely from a sterile surgery.