Help please. When I bite down my back right molar hurts. Are these cavities?

Not necessarily. A sore tooth can be caused by many things. Upper teeth can be affected by sinus pressure. A cracked tooth will hurt under pressure. Gum disease will sometime manifest itself as a sore tooth upon biting. Best you go see a dentist to see what the cause might be.
Possibly. Could also be a fracture or a non-vital nerve. Call a local dentist for an appointment.