What would they do to my teeth if i have a cavitie?

Ok so on my back tooth i have a black spot but it dont hurt.. Ok but i had a filling did on the top but now there is a black spot on the side.. What will they do will they have to do a root canal since i already had a filling in that tooth. And another ? Why when i floss and bring it down on my gums it hurt also it does it when i brush on my gums.. Is there anything to fix that ??