Please visit your dentist.He/she will examine your teeth and act accordingly.it can be a cavity or it can be tartar that get gets accumulated between the gum and the teeth.In both the cases you will have to visit the dentist either for a filling or scaling and polishing

Visit ur dentist as early as possible. the dentist will examine and decide the treatment according to the extent of the decay,if u delay the symptoms might become worse

The thing that you should do is to visit a dentist as soon as possible so that all the teeth can be evaluated for different stages of Cavitation. The treatment will depend upon the extent of the decay and presence or absence of symptoms in those teeth. X-rays might also be required to assist the diagnosis. But delaying the treatment will only make the symptoms aggravate further and some of the teeth might even get irreversibly damaged and in that case they will require extraction only. And you are definitely too young to lose a tooth when there are advanced techniques to save the damaged teeth because there is no better replacement for a natural tooth. You can search for a dentist in your vicinity using Practo and try to schedule an appointment as soon as possible. Regards.