Pupcake

So Jackson is shedding alot in the past week or so. I have an appointment tomorrow with the vet. Jack was just neuterd a week ago Monday and seemed to be fine...then he started itchin and scratching and shedding all over. As far as I understood Shih Tzu's don't shed much, but his hair is EVERYWHERE. When I called the vet to see if they have had fleas that he may have picked them up when he stayed overnight, but they assure me they haven't. She said perhaps it is allergies...to what? dust and mold? I did switch his food, but that was quite some time ago. Could that be the problem? He has aquired an allergy to his new food? It is all natural (Innova EVO) and he has been eating it for at least a month now. Anyone else have any ideas? Or is the shedding normal and he needs a flea collar?