I had this same thing, and it freaked me out. It turned out to be an accumulation of lymph fluids. These eventually resolved through massage and time. It was painless, followed along the scar line, (looked and felt like there was a rope under my skin) and while firm, was not fixated in place except along the edge of the scar. Key factors I think from your post is this is a typical area for fluids to build up and when they do in volume, it can be quite firm feeling. They have no means to get out from where they are, so what you are seeing is the result. It takes a great deal of time for your body to develop new pathways for these to drain, actually all the way down into your chest where other nodes will take over the task.

I think it is too large to be something really bad, and the fact that it arose suddenly, also seems to be a good sign. Please note about the 4-5th image in my personal story located on this page http://www.oralcancerfoundation.org/about/brianhill.htm the rope shaped lump following my scar line.