Does your wardrobe change drastically between winter and summer? i.e. clothes are coming in contact with your neck in the winter but you have lower cut clothes in the summer? Also, do you tend to not pull the sheets or blankets up to your neck in the summer? As someone with allergies to detergents, I found out that my laundry detergent was causing me to have minor allergic reactions. I switched to a hypoallergenic detergent like All Free and Clear and my problem went away.
If that does not help, you may want to see a dermatologist to see if they have any suggestions. Sometimes the answer is right in front of us but it takes the checklist of a doctor to find the answer.
Good luck to you.