What Causes Redness On Cheeks In Children After Having Sudafed?

My daughter is 5. I have been giving her children's sudafed for a week. I noticed a couple of days ago that she had rosy cheeks. My first thought was fifth disease, as the rosy rash is very similar to Fifth's. Then, I noticed that the rash gets worse after getting a dose of sudafed. Is this a common reaction? Of course I will stop giving it to her. Is it an allergic reaction? I give it to her along with children's allegra.