Natural cure for hay fever worse than the disease if allergic to it?

Absolutely! Hay fever (pollen allergy) is an allergy to pollens, which are plant proteins. Many "natural" remedies are derived from plants, and it's not too hard to imagine that some of the plant proteins in the natural remedies could be recognized by your immune system as being very similar to pollens. A classic example is cross-reactivity between ragweed pollen and chamomile.