I don't believe this is uncommon, and believe it could the combination of several factors (if it was simply one or two, the trigger(s) would be clear). May I suggest keeping a general daily log/diary that tracks quality sleep, food, weather, stress, exercise and general happiness level (or any other factors) and noting difficulty of speech on that day and the next? If you're able to plot this, you may see a correlation (but also check for a lag) that could be very helpful to yourself but also to the greater Stuttering community. Science!