Yikes!! I would definitely stop using - I doubt changing INOA shades would give you a different result. It's hard to say what is causing it without knowing - have you ever used regular hair dye before? And did you ever have a reaction to that? It could be the MEA in INOA or something else.... you might be sensitive to PPD as well.

So my advice: Whatever you do, before colouring again I would definitely do a patch test to make sure it's not going to give you a reaction.

Also I thought with henna you weren't supposed to be able to dye over top of it? Just be careful... it can end up turning green or get really dry and damaged...

Just found this article; there's some info here about people having a reaction to INOA so you're not alone: http://www.scientificamerican.com/blog/post.cfm?id=hair-dye-will-debut-soon-in-us-that-2009-12-07

You also might be able to find some suggestions here: http://www.oprah.com/community/threads/57237?numResults=15&filter=newest