I've just finished day 2 myself, glad to have someone at my point!.
Well,from what I've read on forums and such could be shin splints? (Could) I'm no expert, but is certainly in the right part of the leg. Could be from a number of things. Uneven running surfaces? The way you are landing on your feet, you want to be landing midfoot, and not on the balls of your feet. Urm some also have mentioned streching before and after running helps it..that's all I know off the top of my head. I'm sure others will have some advice for you.
Hope it's nothing too long term though!