Hello~Ouch, I am so sorry you took a mean bump to your head. I am glad that the doctors have not found anything wrong, but, I know you are concerned about the stuttering. 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. Normally, after a few, folks start to feel better, this could very well stop the stuttering also and the migraines as well.
I hope you feel better soon. God bless you.