Well, unfortunately accidents can happen in this line of work. Inside your dogs nail is something called the "quick" which delivers blood to your dog's nails. It is rather easy to nick this, especially when dogs are moving their paws during the session. Although it may appear serious, it is typically a very minimal amount of blood and can be treated with a bit of styptic powder. If bleeding continues even after you get home, simply apply a minimal amount of cornstarch and a bit of pressure on the tip of the nail.