You may be having defective enamel by birth (hypoplastic teeth )
Or
You must be having self induced weak enamel due to over brushing or night grinding of teeth ( bruxism )
Next Steps
Visit a dentist for diagnosis & to prevent further aggravation of problem
Health Tips
Avoid extreme cold food which may cause sensitivity
&
Avoid hard food substances which may further chip off the teeth
You can also use sensodyne rapid relief or Colgate sensitive tooth paste till you visit a dentist