You might consider speaking to a psychiatrist. It might be nothing, but it could still help to speak with someone. It might be imagination, but if its a mental illness you might need medication to help.
You cannot physically become a demon, so don't worry. Spiritually though, maybe you were a demon in a past life, or its similar to otherkin, but I don't know if that's even possible. Just remember physically you will only ever be human.