I would keep an eye on it. I'm trying to remember back to Gunther's neuter if he got a purple bump before he had an abcess. One of his internal sutures didn't want to stay internal and came to the surface. It freaked me out at first b/c they used huge sutures and I thought he had some internal organs coming out!
As long as it doesn't look infected, it could be something as simple as she bruised or pulled something. If it gets larger though, you may want to take her in to make sure she didn't pop an internal stitch. By now, it shouldn't be a big problem as they use dissolvable for the internal ones and they will be breaking down.