why do i get an instant flow of words with zero stuttering?

why do i get an instant flow of words with zero stuttering? I did public speaking hours ago and when I was presenting my topic I stuttered alot but there were some moments where my brain just shuts down and I instantly relax. My words flow so elegantly but that's only for few seconds(max 1 mintue) then I'm back on the rough terrian (stuttering) Does this happen to anyone and is there a reason why?