That could be a reaction to anesthetic. You have to drink lots of water with electrolytes (Gatorade). Possible cause is that your body is reacting to some kind of chemical. Ask your dentist what kind of anesthetic was used and try to avoid it in the future.
Another reason could be a local infection that have no place to go (after the roots were filled) but in the blood stream. Your immune system is trying to destroy the bacteria and the toxins are released in the bloodstream. In addition to ibuprofen, sometimes it is ok to take antibiotics prior to root canal therapy, but it all depends on severity of infection inside the tooth being treated.

Kenneth Karamyan