I would maybe guess that your root canal treatment was on a tooth towards the back of your mouth. This procedure does often require you to have your mouth open fairly wide in order to allow the proper instruments to be utilized. It is not the typical posture for your jaw and muscles that control it and therefore these muscles are often sore for at least a few days after a long procedure. Think of what your leg muscles would feel like if you went out and run a marathon without training for it. Sometimes if you have an underlying TMJ problem or an unstable "bite" this type of thing could aggravate it. There are many muscles that you use in positioning the jaw some of which go up to your temple area (headaches) and towards your ear.

The disc of cartilage in your joint can get pulled out of position by the muscles and this prevents you from opening wider. There are various remedies for this and it would be important to have it evaluted in order to give definitive recommendations. I would suggest warm compresses to aid the circulation, antiinflammatory meds if you are able to take them, soft diet only for a few days.

If it doesn''''t start to improve a visit to the dentist may be needed.

Good Luck and remember that most "back" teeth will need to be protected with a crown after root canal treatment as they can be more prone to fracturing.