Antibiotics kill bacteria, and sometimes they kill the helpful bacteria in the gut as well as any infection. Your dog therefore might get diarrhea because antibiotics cause an imbalance in the bowel. If the dog isn't absorbing the goodness from the food because of diarrhea, this will lead to weight loss. However, if the dog was already unwell enough to be on antibiotics, your vet will want to know about the tummy upset. A course of probiotics may be necessary in order to get things back in balance.