Stutter is known to vary in degrees that seem radical. One day you're fine, the next you're just gutted. Patterns might be detected, that might indicate which factors are involved, but it's not terribly mechanical in nature, and that's something we're going to have to get used to. - Stuttering in your piano playing (or other action) is rare. We've heard about it from other stutterers in here sometimes, but it's not common. If I were you, I'd go see a professional of some sort. It's not known to be a typical part of stutter, so perhaps something else can be found out. At any rate, with a stutter, you can have habits developing based off of the stuttering situation, which adds further complexity to the whole thing. Basically, it's hard to tell what's what - but it's worth a shot seeing if some of it can be managed better. :)