I too get a colorless rash on my face for no reason. I am allergic to a lot of fragrances but this last time I had not used any new products. I have found an herbal remedy that works to get rid of the rash in a few hours.

Make strong chamomile tea (4 tea bags in 1/2 cup of hot water, steep 5 min., take out tea bags)
add 1 teaspoon baking soda
2 tablespoons witch hazel
2 tablespoons cider vinegar
1 cup cold water.
Mix well. Soak wash cloth in mixture, wring out. Apply to rash for 20 min, wetting wash cloth when it becomes warm. Apply aloe vera to rash area after the herbal soak.

Hope this helps. I got it from an aromatherapy book but substituted tea instead of essential oils for the chamomile.