I have a feeling that when you bumped your head, you could have done a number on your neck, causing many vertebrae to become misaligned and pressing on nerves, if this is the case, then stuttering
could be one of the symptoms
. I feel that seeing a chiropractor should be your next plan of action. Tell them what happened, he/she will take some x-rays of your neck, this will show any misalignment in that area. Then, they will give you an adjustment.