K.W.
I agree that this could be an allergy...but the allergy could be something in the medication you are giving her. My sister had severe hives when she was younger. We found out the reaction was to the orange dye in the medicine she was taking for an infection! So, it could be something in the Singulare, or something you give her for the cold. Sometimes drs. don't think about the medication causing the reaction!
Good luck!!