Diabetic Cat question

I have a diabetic cat who was diagnosed last week. We have been giving him insulin shots twice a day. His litter mate, a female (both are 8 years old) has been hissing and growling at him since we returned from the vets. They both went to the vets for their regular checkups, so its not like he came home from the vets to a cat that hadn't gone. My question is, can the insulin make a cat "smell" different, that would cause the hissing/growling? My vet had never heard of anything like this. I would love for this to stop as they have always been very affectionate toward each other. The only time they ever did this is when one went to the vets and the other stayed home.