Are relapses common, if so, why does it happen?

Are relapses common, if so, why does it happen? Hi everyone, So, I stutter from childhood and over my life I had good times and bad ones or relapses as I call it. I was in a good one for quite some time, almost a year, people didn't even notice. Upon an oral presentation I got really nervous and I stuttered a lot and had awful blocks, since that it hasn't got down and this was way back in March. I don't know what it makes it stay when I'm no longer anxious or stressed...