If possible upload a picture of your tooth. Firstly, I advice you to visit a nearby dentist and get your x-ray done. As you are saying if it is an extra tooth (supernumerary) and if it is mobile then I suggest you to get it extracted. If it is not an extra tooth it can be saved also depending on how much decay it has.
Next Steps
Visit nearby dentist.
Health Tips
Rinse with luke warm salt water. Brush properly in that area so that it will not decay further. Use floss. Brush twice a day to avoid futher decaying of other teeth.