What could be a possible reason for the knot in my leg?

About 9 months ago I was standing on the steps when a dog, a boxer, ran up them and accidentally hit my leg with its leg. It immediately got a big knot and bruised and was hard to walk. Got it check for a break but of course it wasn't. Now 9 month later, although pain free, I still have a knot and a light blue discoloration over the knot. Any ideas? People want me to go to the Dr but I don't see the point.