Reiki healing can go very smoothly for the new practitioner. That is, until a strange or scary experience happens, such as feeling a sharp or sudden pain as a consequence of channeling the Reiki energy for the client. This is especially since the energy not only goes into the client but out of the client as well.
For example, everything may seem to be going fine with a client and pretty much what you have experienced in healings so far. Then all of a sudden, when you put your hand in a particular and common position, maybe your hands could not move and you suddenly got a terrible pain, such as a very bad headache or a sudden extreme cold, fainting feeling. This can be very frightening, and you may feel like you have to take your hands off your client and ground yourself.
So what happened there, and how do you handle it?! Well, I think you would have had a Healing energy come through you that you are simply not as yet familiar with, so it was scary. If possible, next time keep it going till the bad pain, cold feeling, or fainting feeling stops. If needed, sit on a chair doing the healing, until the feeling leaves your body. Then the Healing is done.
There is no need to be wary of what energy you channel in or out of the client. You are being healed at the same time throughout, and by getting these feelings, you are getting a measure of how long you need to stay in that position. In other words, you know to stay there until this feeling goes.
Flow with it. Observe it. It is only energy. Get a childlike curiosity and be curious about how the bad pain will go from a hard rock in your head to a sponge. Feel it becoming that sponge. Imagine the sponge becoming a cloud. Feel it and notice it dissipating. Be curious about where it will come out of your body. Will it leave, through the Crown? Or sideways, for example?
Also, you can use the pain drain. Have one arm draining into the violet flame, all the psychic gunk energy in your head, into.the violet flame. Till all this gunk is gone. Do not expect this healing position to last only 3-5 minutes. I have experienced such things to go on for half am hour; sometimes even the whole session.
I hope this helps shed light on some questions. enjoy your healing journey and development.