Nobody here will be able to tell you the reason If you worry there might be a connection between this and the kick to the head then definitely get it checked. I think 18 is an unusual age to develop stutter, can somebody please correct me if I'm wrong. I think most of us who stutter developed it as kids/early teenagers and that is very often associated with trauma, I don't know if that would actually be the case for you. Apart from the stutter has everything been alright in your life? Any family problems, self esteem, depression, withheld emotions? I know these may sound like very personal questions and you don't need to answer them here, just something to think about