After having 2 teeth shaved down a bit, because of grinding my teeth, my jaw has been hurting a lot. Why is this?

Yes you can have a core buildup with a crown. The core helps strengthen and anchor the crown to the tooth. 

Hello, my name is Miles. Without knowing which tooth had the root canal (i.e., was it an incisor or a molar?), and without an x-ray and ideally an intraoral photograph of the tooth, this question is impossible to answer.  However, there are some general guidelines that I believe apply....

Hi:It is sometime difficult to exactly spot the origin of the pain.  Tirgeminal neuralgia, missed canal, infection, clenching or grinding can be some of the possible causes.  I suggest to take a Cat Scan of the area to check the root canal tooth more accurately.Best,Dr. Maddahi

Hi:Sorry to hear about your situation.  Unfortunately without a photo it is difficult to judge the size and the contour of the crowns.  It's best to post a photo.Best,Dr. Maddahi

Hi: Good question.  Normally core build up that requires post or pins is an additional charge. Best, Dr. Maddahi

I would recommend seeing your dentist for an evaluation.  Anytime you are having pain with a crown upon biting that isn't a good sign.  Most likely the crown/tooth has been bothered by decay or potentially a crack over the years.  I would recommend an x ray to see your dentist for...