Oxypowder is supposed to cause watery bowel movements, but you should be able to control them well enough to get to the toilet in time. You should take enough Oxypowder to have 3 -5 watery BMs a day. If you are having significantly more BMs, or are unable to control them, you need to cut back on the dosage.
Diarrhea from pathological causes, such as food poisoning, viruses & amoebas etc usually cause forceful BMs that are hard to contain because your colon is irritated by the cause and trying to expel everything as fast as possible.
Be sure you are drinking enough water. Dehydration can cause headaches, as can loss of electrolytes. If you are drinking more than a litre of water a day you should add a pinch of salt or electrolyte solution to the water.
Clarkia has also been known to cause headaches. This could be related to the parasite die off.