It is possible for moderators to see for each user a list of the users having given and a list of the users having received the most votes (up and down) for that user. These lists are partial, showing only the users with the highest numbers of votes given/received relative to that user.
There is a user who has given you an unusual number of downvotes (30 downvotes cast in total, 12 (40%) on you). The other top downvoters on you as well as your top upvoters have much lower percentages (<= 25%). I can not see who received the remaining 18 downvotes and who received upvotes from that user, as the list of the most votes given by that user shows only you, which suggests that no other user has received unusually many up or downvotes from that user.
I am not entirely convinced that 40% is a high enough number to definitely be considered serial downvoting (though wouldn't object to a decision that yes), and I am also not aware of a possibility to manually un-do votes if not caught by the automatic radar. However, inspecting the votes received reveals that the user in question seems to have up to five sockpuppet accounts on this site (significant cross-upvoting and shared IP-addresses). I'll bring it up in moderator chat; quite possibly at least one of the accounts should be put down anyway, which would invalidate their votes.