There is a really great web site http://www.thehorse.com/
that has all sorts of horse health issues you can look up. I use it all the time. There are too many things that can cause lameness and its difficult to self diagnose. It can be lameness in the hoof, leg, hip etc. There are some very knowledgable folks on this site who may pop in and give their input, but in the mean time check out the horse health site. Having a horse veterinary manual for the horse owner on hand is good horsekeeping practice too so you do know when its time to call the vet.