So many questions... Is it possible you could join the site and reply in this thread?
Phone? OS? Version? Was the post nonsensical, inflammatory, routine, what type of post?
Do you by any chance take any sleeping pills? Some of those are known to cause people to engage in activities on an automatic basis with no recollection that they were active.
Side Effects of Sleeping Pills: Common and Potentially Harmful Side Effects
Otherwise, could it be a prank by a coworker?
I'm not sure that an evil hacker would spend the time and effort to take over your viber account and post to one of your group chats. What would they gain?