I haven't been on the synthetic, but I had strange reactions to one of the generic natural meds - switching brands seems to have worked (today!), so you might want to look at that. Often it's the binders and not the active ingredient that's the problem. Do you have other allergies?
Tumblina's suggestion about allergy to the binders, dyes, and fillers is a good one. Hypo people are sensitive to such allergens anyway, so it's entirely possible that there's something about that brand that's making you itch and break out.
Report this reaction to your doctor and ask for another kind.