Are you part of the voting club from Steem-Bounty (https://steem-bounty.com/services/vote-club)?
If so have you checked your logs?
They let you check all the votes you have performed. You can compare and check who has upvoted you as well as who you upvoted.
Below are the votes I performed.
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Downvoting a post can decrease pending rewards and make it less visible. Common reasons:
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