My teeth became sensitive after sucking on a lemon. why does this happen?

Acid: Lemons are very acidic and sucking on them just causes erosion of tooth structure, tooth structure you can't get back. Unfortuntely this is a very dangerous habit. As you erode tooth strucutre you can get issues involving the nerve of the teeth. This may be the sensitivity you are having.

Acid erosion: Citric acid, the "tartness" component of lemons and other citrus fruits is acidic enough to dissolve the calcium mineral content of enamel, dentin and cementum of the tooth. If enough crystal tooth structure is removed from the root, the sensitive internal parts of the tooth can be exposed to changes in touch & thermal stimuli. Fluoride increases acid resistance. Stop the damage with diet change.