I’ve been lifting weights for 15 years and have had periods of time when I wasn’t lifting. I have never seen a correlation between stuttering severity and weight lifting. In fact, my worst periods of time regarding stuttering were when I wasn’t working out. I wouldn’t worry about. If anything, I’d say working out will help your stuttering because it tends to lift your mood. In my experience, attitude and mental health have, by far, the most influence on fluency. Just my two cents.