Hydrogen peroxide is pretty potent stuff...you may be experiencing a lot of die-off from it that is causing you to be symptomatic again...or it may be creating a lot of free-radicals...oxygen therapies can be a little tricky and need to be carefully balanced with antioxidants.
As for your question on colonics...I'm putting together a 'general info' post on different types of colonics...should be posted later today.