In my Reiki I initiation, I had memories come up and float away, but never anything bad or scary. Reiki can bring up some past issues that are ready for a cleansing. I could only suggest that you keep using the Reiki on yourself to complete the cleansing. Be sure to be drinkning a lot of water. It could be toxins coming loose in your body and you want to get them on out. I will tell that Reiki energy is ONLY loving energy. There is no harmful energy in it at all, if it is true Reiki. What you are experiencing could be something else. I pray that you will go to your heart and focus on God and Purification. It is very powerful to ask for God's assistance and intervention into any thing that is not for your highest good.
I hope this helps.