Dentist says I need a root canal just 3 months after my new crown was placed. I feel he screwed this one up! Is this normal?

Hi, Emma.  Sorry you are having problems with your tooth. Thanks for the photo, it helps. You had rather large deteriorated fillings in the tooth prior to the crown and it appears that a crown would help you strengthen the tooth. Unfortunately, the tooth had prior trauma to the tooth, decay and large filling. Sometimes the nerve just gets inflamed after a crown preparation and it can't repair itself. Thus a root canal is the only way to proceed. Your dentist adjusted your bite because sometimes this will help your tooth recover. You might get a second opinion is helpful by a root canal specialist.

Hi:From the xray it appears you have a deep large fillings in the tooth.  Every time the tooth is prepped for a crown there is a small possilibty of nerve not recovering from the dental procedure and a root canal might be needed.  Because of the large filling this possibility is even greater however, no one really knows if you will need a root canal prior or not.  In terms of bite adjustment and integrity of the crown, a clinical exam is necessary to judge that.Best,Dr. Maddahi

99% of the time the need for the root canal has nothing to do with a treating dentist.  Simply having a filling placed can push the nerve "over the edge" and need therapy.  The "cause" was the decay and not any treatment.