I think the psychic healer is BSing you. There are no triangular reiki symbols so I don't know what got into your forehead assuming she is being truthful with you.
I am a reiki master and I know a couple dozen other reiki masters. None of us were possessed or had any problems related to their master level. How were you attacked after your first level?
Reiki masters make no claim to being psychic so I don't know why your healer expected them to be.
Was your teacher a long time reiki master? Who did they learn from? How was their energy in general? Did you feel OK around him or her even when reiki wasn't involved? They may have been doing something other than reiki when you were getting attuned if in fact you did have other symbols in you. Or those may have been left over from something lifetimes ago.