In general, flag for moderator attention and explain the issue. We do intervene in cases of serious misuse of the voting system and will invalidate votes and suspend users, if necessary.
Users are free to vote on posts based on their own criteria, the major restriction is that targeting posts by a specific user with votes is not allowed. Downvoting a lot of posts from a specific user is prohibited just like creating sock puppets and using them to upvote your own posts.
In this specific case there is no indication that any user is targeting you specifically with downvotes.