May be infected... If the tooth does not hurt at all during the day, or with a stimulus such as heat, cold, or chewing, you may have a sinus problem. It also may be in the beginning stages of nerve death....Kind of hard to say. Keep a close eye on when it hurts, and if at any other time it bothers you, get to the dentist pronto! if it is only at night, take a decongestant before bed and see if that helps.