Hi, Michelle. It's been quite a bit since I've lurked or posted around here, but I was drawn to this post to provide some insights on the matter.
I'd like to assure you that it sometimes happens. I've felt aches, pains and exhaustion sometimes when treating other folks, or even myself.
There have been particular cases in which the person has needed quite a lot of energy. In those instances, I've felt lots of pain in my arms, lots of vibration in my hands, and sometimes even in my legs and feet to the point that I could easily lose my footing if I am standing up. My take is that a great deal of energy is flowing through our channels, and this significant flow is the one responsible for causing those aches and pains. Once the session ends, those aches and pains diminishes, and sometimes the exhaustion is replaced by quite an energetic boost, kind of like how you feel after a power nap.
It is sustained with Reiki that as the Energy passes through us as practitioners, it works with us as well before it goes to the patient as you work on them. Therefore, another take would be, in addition to the flow of energy the other person needs, that the energy could be working things that need to be worked within you. It is said that whenever the patient or the practitioner feels pain while giving/receiving a session, the energy is working on the cause of the disharmony, instead of just energizing the area. It could be breaking a blockage in any of the persons multi-dimensionality, in particular the physical, emotional or mental bodies, which in the end all boil down to energy forms with different vibrations and densities.
You could feel even things such as nausea, upset stomachs or the like, when you're treating someone that has been quite affected by negative vibes that have manifested in illnesses in their body. Such has been the cases, for me, with folks with fibromyalgia, cancer, or in the final stages of passing on to the other side, especially when affected by terminal illnesses. When that happens to you, you can step back a few steps to regain your energetic bearings, and you'll notice that all those ill effects will fade away on their own. Note that it is not because the other person is passing their energies to you; it's more like that because of the empathy and connection created between practitioner and patient, you might feel such things as their own healing process takes place. In the end, this is what the Energy, regardless of its manifestation, does with the person. It awakens their own self-healing capabilities that may have been otherwise hindered or rendered asleep because of the energetic conditions of the individual.
I hope this sheds a bit of light on this. Sorry that I am posting this almost a year later.
Blessings, and warm regards, to you and to all, with my constant gratitude and love.
Francis