Mild pain after filling a cavity is normal. This is often seen when glass ionomer materials are used for fillings. Glass ionomer materials expand in volume within 24 hours of mixing, which can stimulate the dental nerves and cause pain. However, the pain should gradually decrease or disappear after 24 hours. If there is severe pain after the filling, it may be due to the filling material being too high or the tooth decay being too deep, reaching the dental nerves and causing nerve pain. This situation is abnormal and requires root canal treatment.