Today i received a notification which says that one of my posts got one "unupvote".
upvote was removed from one post 1 time"
What does this mean? because i think an user after voting can't remove his vote or can he?
Votes are locked 5 minutes after vote is cast . Votes, that are cast, can be removed (either unupvoting or undownvoting) only when that particular question or answer is edited.
I saw your profile and noticed that you edited that answer yesterday and after editing the answer that particular user, who had cast his upvote, removed the upvote.
For more details visit these meta questions Undoing an old vote cannot be recast because vote is too old and Why do votes get locked?