I got a forced update after I run git pull --rebase

I can't reproduct my issue, this is my first time, but my colleague
came across this issue several weeks ago.

After I pushed my commit to git server without rejection. I run git
pull --rebase, then I got a forced update, and my last commit is
missing.

I have asked a question on StackOverflow, there is more details about
this issue: https://stackoverflow.com/questions/49164906/why-git-pull-rebase-results-a-forced-update

I have no idea about this.

I'm using Ubuntu 16.04.4 with Git 2.16.2, PyCharm IDE.

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Sincerely,
Tian Zhen