Pain is one of the most unpleasant experiences to affect living creatures. As humans, we can tell other people when we are in pain. It’s much more complicated in animals. They cannot directly communicate their sensation of pain to their human carers, and the detection by humans of pain in an animal can be difficult. Great care needs to be taken to ensure that animals do not suffer unnecessarily.
Humans have been very arrogant about this subject in the past. In medieval times, it was the common belief that animals could not feel pain. It was believed that animals did not have any consciousness, and that they functioned by automatic reflexes only. In the seventeenth century, Rene Descartes and his followers deliberately beat animals to death to “demonstrate” that they had no feelings.
Please note that I am unable to answer veterinary questions in comments. If you have questions or concerns about your pet's health it is always better to contact your vet.