What could cause sharp pain in tooth after eating a grapefruit?

Tooth decay: If its just cold sensitive it could be decay or root sensitivity. If it goes away quickly it is most likely not too deep and a filling will suffice. If it lingers for more than a minute, it may be more involved. Possibly need a root canal and crown. See a dentist. It won't go away without fixing it.

Cavity: Could be a cavity. When you have dental decay (aka caries, cavities), the enamel is compromised and dentin or pulp tissue can be exposed to the outside environment. Dentin is the layer of tooth under enamel. Dentin has tubules that can elicit pain with sweets or temperature changes. Please see the dentist for a diagnosis of the tooth in question.