Can A Cavity Be Filled With A Veneer On It?

Have a veneer on my upper rt eye tooth #3- tooth has alot of bonding filling.Noticed today between this tooth a small brown stain at the upper part near the gumline-hard to really see it bec it is betwn my teeth. Have a little dental mirror. Is this a cavity? looks like it is next to the bonding material.Just a small dot. It may just be a stain hopefully.I am worried it is a cavity and how in the world is my dentist going to fill this and not go through my veneer?No pain. wrapped on bottom only