I'm not into Reiki either.
Everytime I do energy work, I tend to get crazy cold afterwards. I even once tried to put my hand on the surface of my freezer, my hand was more cold than it ! To me it's normal. A bit imbalanced maybe yes, I would agree.. but then again, are you more the "giving type" ? I suspect you are.
After all, when one's full of energy, they generally feel heated ! So it's only normal after energy work, you're depleted, and cold. I'd say there isn't too much to worry about. Eating something you really like afterwards is a good idea, while respecting your diet of course.
Be well !