Have you reported it? In case you haven’t already found it, here’s the link you need: https://medium.com/policy/mediums-copyright-and-dmca-policy-d126f73695. Everything you need to provide to Medium is outlined there.
I have no way to actually research your claim. There is no history in the Wayback Machine for the Possibility Change site (I’m not saying I don’t believe you). If this work has in fact been stolen from you, it should be fairly easy to prove. I’m sure that the owner of the Possibility Change site would be able to provide a dated archive to prove the actual date you originally posted your story. I would imagine that’s all you would need to provide to Medium in the way of proof of your ownership.
None of us want to see our work stolen so I hope you follow-up and get this resolved.