Is there a filling in the tooth, could be it's leaking a wee bit and needs further investigation. If it's not filled or the filling looks fine, then it could be dentine sensitivity. Long technical explanation follows, skip the next paragraph if you want :D
A tooth is made up of an outer shell called enamel, and an inner substance called dentine. Dentine contains little tunnels that link the nerves on the inside of the tooth to the junction to the enamel on the outside. These nerves are a bit simple minded, they only carry pain impulses if anything stimulates them, hot, cold, sweet, or pressure, they trigger a pain response. If the tunnels (we call them tubules) get exposed to the outside then you get that response. In some areas the enamel shell gets quite thin. One of these areas is where the crown of a tooth meets the root. The root is covered with a material called cementum. The cementum is soft and easily worn away. Most people have a butt join between the enamel and the cementum, others have an overlap, some have a small gap.
In your case, what might be happening (can't say for sure without looking) is that there is some dentine exposed, as we get older, our teeth come further out of their sockets in order to compensate for wear of the crowns, other times we lose a little of the supporting structure due to gum disease. You could have a gap between the enamel and cementum, which allows the dentine to get exposed and stimulated (oooer :o) hence the pain.
Sensodyne works by blocking off the tubules, but doesn't work that well used as a toothpaste, it works a lot better by gently massaging it into the sore spot with a finger. Do NOT rinse it off with water or mouthwash.
There are other products your dentist can use if this doesn't do the trick. None of them hurt to apply :D