According to your description, you should be allergic to dentin. Eating grapefruit can only show that it has caused your dentin allergy. You haven't noticed that your teeth are not very healthy. Generally speaking, the most typical symptom of dentin allergy is that when you brush your teeth, you will feel short and sharp pain when you are stimulated by cold, hot, sour and sweet. For example, early dentin hypersensitivity should be brushed with anti-allergic toothpaste for two weeks. Go and buy a comfortable anti-allergic toothpaste. My colleagues around me have the same symptoms as you, and they all use Shu Shi Da.