Stuttering only when thinking

Stuttering only when thinking I've been stuttering since the 4th grade and it's significantly improved over time, but I still notice that I stutter mostly when I'm thinking about what I'm saying. Like in german class, the stuttering is much worse because I really have to think about everything I'm saying, but when I'm playing a sport of doing something like a routine, there's no stutter. Does anyone else experience the same thing or know why this happens?