Fur loss and discoloration in pug?

I have a 10 year old black pug and recently his fur has been turning a lighter hue and coming off in massive clumps. It's not his whole coat, just random patches throughout - mainly near his rear legs and collar. This happened in the past when I used a new dog shampoo on him, but I have not bathed him myself or changed his diet in over a year. Last time I took him to the vet for this they said not to worry if it did not seem to bother him... but I am just curious as to what is going on?!