Reiki attunements are a strange thing. When I got mine, I had assumed that reiki was the same as qi gong, prana, etc…but it really seems to be its own thing. Looking back on my experiences, it seems to me that the body takes a very gradual process in integrating this energy with the electrical system of the body. Its far too intense to introduce immediately, and my teacher warned me of trying to do to much to early. Apparently it is inexhaustible when fully mastered, but getting to that point takes time, and you kind of have to retreat for awhile during the metamorphosis. I didnt listen and started giving myself constant nosebleeds, full-body cramps, and causing absolute chaos in my life.
Long story short, my suggestion would be to take some time out from things and just take it easy. Youre going to be undergoing changes whether you like it or not, and to attempt living your live as usual during this wouldnt make sense. Im not saying turn into a hermit necessarily, but focusing on your internal world more than the external, at least for a little while will help immensely. Passing out is always a little concerning though, so I wouldnt rule out going to the doctor if youre concerned.
To be honest I havnt been the best student in this field, but these are the things that have helped me, or that looking back, I wish I had done. In fact, i think ill go give my teacher a call right now.
If you really want the best guidance through any of this, I would try evoking Usui himself. His picture alone is quite powerful. Many practitioners I know are very fond of Quan Yin as well, she seems to have a very calming, mellowing presence.