After the contest is end, I found that there was a resubmission in problem B, but I've never done it. The code is definitely not written by me, so I wonder if it is a bug in Codeforces or just someone else broke my password and signed in to my account.
I have now changed my password.
Hope it could be fixed soon
UPD: It has been fixed.
iT iS aN eViDeNcE oF cHeA-I got the same issue on my facebook account used to participate in FBHC. Our handle can also be used as the username when we log in, so maybe someone just guessed the right password.
Facebook has the same issue, we can use the phrase after "facebook.com/" as an username when logging in:
Hope this never happens to anyone.