Was it a dream or did it really happen?

It could have been very real. It really doesn't make sense how such a young person could know about such things beyond their years. I would suggest seeing a psychiatrist. They might be able to help you look deeper and figure out exactly what happened and help to uncover more of the memory. A lot of times, memories from long ago become foggy and after a while you can't distinguish that from what really happened. Good luck!