Many people believe in reincarnation, so the idea that their souls (or spirits, or whatever you wish to call them) lived many lives in the past isn't a completely foreign concept. Undergoing past life regression can help bring up memories of these former lives, some of which are quite scary. The creepy stories compiled here from Reddit, all of past lives and odd memories, may help you come up with a few theories of your own.
Made the Baby Go Away
Not necessarily a past life, but my friend had a miscarriage before she had her son. He had no idea about her miscarriage, and was about 6 when he said the following: "Remember the baby you had in your belly? I made them go away so you could be MY mommy."
This child also saw spirits in mirrors until he was in about 5th grade.
Memories of a Car Crash
I still have a very short memory of a dream, although I can't remember actually having the dream, just the dream itself. It starts with a women who my dream-self identifies as my mother screaming for my safety. I turn around from whatever I was doing on the dirt road in a busy city to see the terror on her face. Then something crashes into me. I immediately get upset about the extremely consequential accident - I had plans and strategies for this life, and now I had to start over.
Based off of the angle of my vision in this little scene, I can guess my height and say that the dream figure was probably around 6 or 7. Also, the fact that it was a dirt road in a busy city makes me think it was India or something, although I can't say for sure what the skin tone of the mother was (it might have helped). Also, this sounds like a fatal car crash, but the road was a dirt road, which seems really weird.
I have a slight memory or looking out of a window at a small walled courtyard, feeling some pride about it and the dozens (or maybe up to 100) people who used it, many of whom where my offspring, and smiling a goodbye blessing.
I remember as a very small child spending a long time trying to remember why I had chosen to be born this time as a girl, since everything that I was learning about the world seemed told me that it was categorically better to be a boy instead. Oddly, I have no idea what gender I was in the two memories of death.
I also remember thinking that it was kind of unfair to compare me with some of the other elementary school students, because I could just remember things, while they were learning them for the first time.
I also remember trying to ask my mother about these memories, but being careful just in case she didn't believe in life before birth. She didn't, so I couldn't ask anything more detailed.
Now You're the Dad
My son said something to me when he was 2-2.5 years old, we were on top of the stairs while I was fixing something (can't remember what) and he said to me "Funny. Me were daddy, now you daddy"... I'd be about 29/30 by then and my father died when I was 11... He only said it that once and I've never told him... Never told anyone apart from my best friend about 20 years ago...
They Helped Fix Grandpa and Somehow Spoke German
When my daughter was 3 she told me several times that she knew me, my sister, my mother, and my dad before she was here now. She said all of us women wore white dresses and worked in a white tent with red crosses on it. She said we all helped fix Grandpa after he got "poomed." (Her word for gun sounds when she was little.) She also told me that we have taken turns being the mommy. She said I was doing a good job this time. I've actually accidentally called her mommy a couple times when I was really tired.
My little boy spoke what sounded like German as a baby without hearing it before and when he was older we got a German babysitter that was surprised we only spoke English because he was speaking low German at her house.