Restlessness:
Pained dogs often become restless to the point of becoming incapable of lying still. They pace excessively and change positions frequently, unable to get comfortable. Dogs in pain also have altered sleep patterns due to this restlessness and may either struggle to sleep or end up sleeping too much.

Appetite Changes:
Pain, either due to external or internal injury, often causes appetite changes in dogs. Although a dog having difficulty eating is most likely to be suffering from dental pain, reluctance to eat due to loss of appetite is also found in other conditions causing pain. In addition, certain painful conditions and diseases can also affect the amount of water a dog drinks.

Aggression:
Aggressive behavior and antisocial tendencies are one of the most common signs of pain in dogs. A dog in pain may hide away to avoid you and not enjoy being touched anymore, even becoming aggressive in response.

Vocalization:
Dogs in pain will be more vocal than normal to let you know that something is wrong with them. A dog in pain when touched, usually yelps to convey its discomfort, but it may also growl and snarl, often without an apparent reason. Pained dogs also tend to howl more frequently and can become quite aggressive when pushed.

Excessive Grooming:
Another sign of pain in dogs is sudden excessive grooming. Dogs lick their injuries instinctively to remove debris and prevent bacterial infection. So, if your dog is licking a certain spot excessively, there is a good chance it is injured, and you should try to reduce the associated pain.

Altered Breathing:
Altered breathing is also a sign of pain in dogs. When in pain, dogs may have a breathing rate faster and shallower than usual. Some dogs also pant when in pain, so if you see your dog panting heavily despite not having undergone any physical exertion, do not ignore it. It is often a sign of something serious.

Affected Mobility:
Any physical injuries of the limbs affect mobility in dogs. If your dog develops sudden hate to exercise and seems to struggle with previously easy tasks like getting up or climbing stairs, you should consider the possibility of pain and soreness due to a joint or limb condition like arthritis.

Posture Changes:
Dogs also tend to change their postures when in pain. For example, some dogs become more rigid and assume a hunched posture when in pain, whereas others try to stretch out by moving their hind legs into the air while their front legs are firmly planted into the ground. This position is known as a 'prayer' position and is most common in dogs with abdominal pain.

Muscle Twitching:
Serious conditions like food poisoning, pancreatitis, and kidney disease can often cause muscle twitching and tremors in dogs. So, if you see your dog shaking, don’t jump to the conclusion that it's shivering because it's cold; try to determine a deeper problem.

Swelling and Inflammation:
Swelling and inflammation are some of the most prominent and major signs of pain. Excessive inflammation can even cause pain directly, while any physical injury, infection, or allergy can lead to swelling and inflammation in a dog and is an obvious sign of something being wrong with your pet.