I wouldn't like to advise really, it might be something to do with the shoes (do you have a pair from a specialist running shop?) or it could be something else.
If it was on the shins I was going to suggest that it might be shin splints, but it isn't.
If you haven't got the shoes then you really do need them (they need replacing every 400 miles or so), if you're okay on that front then I think I'd try the GP first to see if it might be the veins. If you still get no joy, then think about a physio.