Even without CRPS people can have off days when they are tired. It could be the CRPS or the medication side effects (only likely though if you have just started a new drug) or it could be nothing at all. I wouldn't worry about it - it is only a problem if it starts to affect you on an ongoing basis or becomes a significant detriment to your quality of life. Then you should seek medical advice. The more you worry, the worse it will make your symptoms so it is best to roll with it and see how things go. Take care.