Severe pain on one side of the mouth, especially while chewing, is usually due to a dental problem such as tooth decay, gum infection, or a wisdom tooth issue. While a pain reliever can help temporarily, it will not treat the root cause. For short-term relief, you can take a painkiller and rinse your mouth with warm salt water while avoiding chewing on that side. However, since the pain is significant and recent, it is very important to see a dentist for proper diagnosis and treatment, which may include cleaning, filling, or other procedures depending on the cause. Do not rely on painkillers for more than a couple of days, and seek urgent care if you develop swelling, fever, or difficulty opening your mouth.