Strange Trap Problem

DB and BB shrugs are my best friend.
so today, i figured, since i do OH squats, i might as well do OH shrugs
i was in for quite a surprise
i tried it with DBs
my left side was fine, perfect ROM, i could elevate and descend normally
but my right side…get this: i couldn’t even “reach” or extend my arm. its as if it wouldn’t move or i had no muscle/nerve endings/whateverthefuck there whatsoever.

what the hell is going on? is it a muscle imbalance? weakness? flexibility issue?

*bump
sorry

exactly, only for my right side though, my left seems to have no problem