What Causes Red Rash On Neck And Face?

i have a red flat rash that appeared on my neck this morning and throughout the day it has gotten redder and has slowly spread further down my neck and face. The spot that appeared this morning looks dry and at times the rash areas itch. My neck has itched for the past few days but the rash just appeared this morning. There has not been any changes within my activity, food, etc other than I started taking Vitamin D-3 as of Saturday (this is in addition to my regular multi-viatmin)