See What is serial voting and how does it affect me?
Basically, there are systems in place that prevent serial voting. For example, someone may find one or two of your answers as really good and upvote a whole bunch of others without reading1. On the other hand, a malicious user may give you many downvotes. Such strings of serial upvoting or downvoting in short time intervals get reversed within 24 hours.
1. On SE we vote on the basis of the merit of the post, not the user (though the merit of the user can be a factor). Blindly upvoting a user's post goes against this.