This is a hard question to answer because I don't think anyone can know for sure.
I believe that muscle cramping can sometimes be a SM symptom, I have seen it in affected dogs that have been sleeping and dogs that are exercising, but there could easily be other explanations for what you saw today.
This is one of the problems with owning a diagnosed SM dog, knowing your cavalier has a syrinx can save them some suffering because you are alert to subtle changes in their behaviour that otherwise would have been missed, but it can also make you predisposed to see everything different that they do as being SM related.
SM means you live with uncertainty. All you can do is mentally note the changes and wait and see if they are repeated often enough to be seen as a 'symptom'.
I know how hard you are finding this and I do understand. Harley & Ebony are such lucky cavaliers to be with an owner like you that will do everything possible to keep them comfortable.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi