Personally, if I were you I would call your farrier, because it could be a stone bruise or abscess, or a stone bruise that has abscessed. You could try soaking the hoof in epsom salts and warm/hot water (but not too hot obviously) and see if it makes a difference.
The reason I say farrier over vet for this particular lameness, is because you have noted no problems with the leg itself, so my guess would have to be the hoof, and IMO the average farrier deals with a lameness better than a vet, and costs less too. Who is your farrier?
I can't remember exactly where you are, but my farrier is Bruce Bryan, who unfortunately for me now lives up in Hamilton and does horses there (I'm just lucky enough he comes back down every 6 weeks or so for his clients down here) I have never had a lame horse while using him, and every abscess I've had has been fixed just like that with him, he's really good.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms