What Causes Severe Headaches After Jaw Surgery?

I had jaw surgery 3 weeks ago, my bottom jaw was moved forward 10mm to stop my sleep apnea, which it has done. The problem at the moment is i get such severe headaches. I am taking Veniafaxine and Tamoxefin but need something for the headaches, i am taking Advil 600mg every 4 hours and 500mg of Tylenol every 4 hours alternating ( so every 2 hours i am taking one or the other) my headaches are still awful????