About a month after having a root canal two teeth next to the tooth that had the root canal are starting to hurt after chewing. what is causing this.

Torgue: If you are still missing a tooth, you may be putting too much strain on the adjacent ones leading to the pain. Another issue with a missing tooth is that the adjacent ones would shift (usually forwards) thus changing the dynamics. Contact your dentist if problem persists.

More info needed...: Your age, the condition of the adjacent teeth, periodontal issues, your bite, stresses from a partial denture, etc, etc. See the dentist who treated you for a follow-up.

Sore teeth: Teeth may be shifting, especially if there any missing teeth. You may have a gum/bone issue, fractured tooth, hyperocclusion, etc. you will have to see your Dentist to find the cause.

Possible grinding: It's difficult to answer without seeing your teeth, but there are two possible answers. A common cause for tooth sensitivity is clenching and/or grinding your teeth when you sleep. It can cause chipping or cracking of your teeth; however if the root canal you needed was due to cavities, it's possible that you have a similar problem with the other teeth due to a high cavity rate. See a dentist.

Slight Movement: The most common problem is that these teeth have slightly moved and are in a different biting relationship to these teeth they are biting together on. The simple 1st step is for the Dentist to adjust the biting relationship of these teeth. Your Dentist needs to see you to make this adjustment and to evaluate the area.