Immediately after i got the crown, my tooth started paining so my dr did root canal through the crown and filled it. why she did not put a new crown ?

Reasons vary: It is not uncommon that recent root canal therapy remains painful to the tooth. The tooth may need as long as a year to heal from the therapy, so it is common that a dentist simply fills the hole and monitor before placing a new crown. The tissue under the tooth could swell and the crown may need to be reshaped, so a new crown later may not be a bad idea. Ask your dr about your specific situation.

Less cost and easier: There is usually no problem with doing a root canal through a crown as the opening can be patched successfully. Unfortunately your dentist would have charged you for the new crown and is trying to save you the time of getting a new crown and the expense.

Not necessary: The small opening made thru the crown can be filled and bonded w a porcelain colored material and in most cases that is fine!