A bit strange that the teacher assumed that you hadn't authored it yourself.

A short anecdote.

I remember back in the late 1980'ies, I used to connect with BBS's and had online chat with friends. One guy used to write jokes and share with me and I would also share him the jokes I had written.

Then one day in class, another friend of mine who was also into BBS's, shared a list of jokes from our mutual online friend, and he read them out loud.

When I heard that one of my jokes was included, I mentioned that I wrote it (I didn't expect people to find it that funny).

The hilarious backlash was that some angry person started saying that I was trying to steal the credit for a joke that I myself had written some weeks before.

The moral of the story is that information is local. What people hear first will often impact their perception.