Should there be a limit of downvotes a user can use on certain other user in one day? Because at the moment, all my reputation is shown to be -2, and this is not because of bad posts, because even my old and non-active posts have been dug up and downvoted in one spate. Obviously someone who searched up my profile and down-voted on all my posts. I repeat, should there be a limit of down-votes a user can use on another user in one day?
If a single user is doing it (someone does seem to have it in for you today), then it may be automatically detected and removed as part of a daily statistical voting anomaly removal process. See here for more.
when you have a number of different posts (answers or questions, old or new) all voted down in a single day, that seems suspicious. But if there are a number of downvotes on single posts then that is probably sincerely not popular by more than one person. I see both kinds of activity on your questions (yes, the first kind looks weird). Since you have a lot of positive activity on most questions, I'd take the multiple downvotes on a single post as something is wrong with that particular post and try to learn from it (like in the 'german vs germane' question title, which you did).
there is a high cap on the daily rep points earnable. There could easily be a cap on the other side for number of points you can lose (but I don't know if this is the case). You can only lose points by getting downvoted, or downvoting others (and also by others changing their upvotes and accepts if you edit your posts but that in a sense just means you didn't get the points in the first place). Whether, there is a low cap, I think that the current down rep values and ability to vote limit the damage that can be done.
despite that, what do you think a good low cap would be? -10, -40, minus something else?