Hope this helps,
Jossi Stokes, DDS
Number 2 it is possible that it was really deep and too close to the nerve which then keeps the sensitivity likelihood higher in those cases a dentist usually places and under laying material to prevent your nerves from irritation
Number 3 it is possible that you cavity was too deep and that upon doing work on that tooth it may have gotten infected by the bacteria that has caused your deep cavity and in that case you will require a root canal or an extraction
There could be an infection starting and my need a root canal. These are the most common reasons.