Does everyone have random off days?

Does everyone have random off days? I don't stutter when I go to a grocery store or call customer service or with very quick conversations. But when I have an elongated conversation for more than 5-10 minutes then I start stuttering. Also I have off days, some days I don't stutter at all and some days I can barely get a word out. I don't observe any difference between these 2 moments. Is this how it is for everyone or is it just me?