It's possible that if this is from dope rather than pharmaceutical use, it may be from a certain cut that's being used that's not completely agreeing with you. We have to look at a few things in order to see if this is the case.
1.)Are these isolated incidents that have been happening recently? From the same dealer, similar batches?
2.)Does this happen while you are on pharms like high dose oxy, opana, dilaudid, morphine, fentanyl, etc.
If the answer to the first question is something like: Nope...this has been happening for a while now, from multiple batches of dope, from different sources, which may not share a certain cut which may be causing the issue.....and/or the answer for the second question is : Yes, it has happened with pharms....then-
my opinion would point to your experience with these sensations as being coming out of mild to moderate nods, yet not realizing that you were even nodding to begin with, and you feel the jerk/spasm of your body readjusting and reacquainting itself with normal level of awareness...a.k.a snapping out of a nod. I've seen this happen to people I've used with, and I've experienced this a few times back in my active using days. It's hard to discern individually, and it's easier to realize the issue when you are with someone who isn't as high as you are, so they can visually monitor you and see what happens immediately prior to these spasm episodes.