The definition of a relapse according to the MS Society is “the appearance of new symptoms, or the return of old symptoms, for a period of 24 hours or more – in the absence of a change in core body temperature or infection”.
I’m like you though, I’d only bother calling my MS nurse if it was something significant. Other things like numbness tend to fluctuate with me. So even if it lasts more than 24 hours, meaning it’s technically a relapse according to the definition above, I wouldn’t bother doing anything about it. I’ve got enough experience of my body now to know something like that will go back to normal after a bit.