I would think the most likely cause would just be irritation from the stitch. However, do keep a close eye on it. When Abby was spayed a few weeks ago, her stitches got a little red around the edges...then slowly progressed to big, greenish blisters!
Come to find out, she is actually allergic to the stitches themselves!
How bizarre is that!?!? She had to have an IV treatment, stay at the vet during the day for 3 days and had to take antibiotics.
Again, I would thing it's just irritation, but you never know so keep an eye on it.