The same thing happened to me a couple of years ago. It started when I was around 21 and it was almost like I just woke up with a stutter. I've hit my head a couple of times, but not hard enough to cause damage, at least I don't think? But overtime I've discovered that my stutter usually happened when I got nervous or had anxiety. This went on for years and I still stutter sometimes now, but not as frequent or as bad as I used to. It's very strange. I've thought about getting checked out by a doctor but I'm honestly too scared to. I would suggest that you do though. There could be a psychological reason behind it or there could be something going on physically.