Itchy rash on neck from Differin Gel (Adapalene 0.1%) - what can I do if I still want to keep using it?

I am experiencing an itchy rash on both sides of my neck which started within days of using Differin Gel 0,1% Adapalene. (I only use it on my face and jawline, not on my neck.) Might I be able to change my routine to alcohol-and-oil-free moisturizer and face wash and the rash will go away or is it more likely that I am allergic? My face handles the Differin gel very well, without a rash. I have not been applying sun screen to my neck and ears since I don't apply Differin Gel in these areas.