Why does my jaw hurt sooooooooo much after a root canal treatment?

May be usual, but... There generally is pain after the local anesthetic wears off; the dentist should explain and warn about that depending how invasive the procedure is, whether he had to clear out infection, or abscess at the root. He should also give you a script for pain medication appropriate to the procedure, and appropriate to his knowledge of your pain threshold and ability to handle pain. Call, get advice.
Staying open. You jaw can be sore for some time after endodontic treatment. Often this is caused from the jaw muscles themselves being stiff and/or tired from having been in the open position for an extended period of time. Luckily, this usually goes away on its own.
Depends. Probably had an infection present. Jaw can hurt due to the root canal or due to having your mouth open an extended period of time. Consult with your dentist.