I'm going to be totally honest, I don't think Oxypowder is the best product for everyone. In fact it is the only oxygen cleanser that I have read about people having allergy/sensitivity responses,and kidney reactions to. Also, Oxypowder is not like other oxy products in that it provides a high does of magnesium and is not safe for long term use.
As for the symptoms, it sounds like you are experiencing an electrolyte imbalance and possible sensitivity to the product. I recommend switching to something safer like homozon. If you can't get it where you live call the oxy-store (www.oxystore.com) they are very knowledgeable and could probably guide you to something in your area.
If you decide to continue with the Oxypowder , try supplementing with an electrolyte drink and see if that helps. If it does not, you could try lower doses, though because you have fibermialgia I would not mess around with it if you think you may be sensitive to it.