What if someone posts my original lyrics in the post bar and says it's my own poem?

I suggest you protect the copyright of your work as soon as possible. You can perform copyright registration on the MORP Music copyright registration Platform of China Music Association.

After copyright registration, you can get a legally binding copyright certificate, which is legal evidence. In the event of copyright disputes such as infringement, you are well-founded and will not be so passive as it is now.