Chemical sunscreen would be my guess- an allergic reaction combined with sun exposure. I suddenly got awful rash and hives one day after wearing it out in the sun. I had to switch to the non-chemical sunblock which uses only titanium oxide and/or zinc oxide.
Just one possibility.
The reason I say this is that neck and shoulders are the most exposed to the sun- look up allergic reaction to sun exposure- which has similar symptoms....and sunscreen can aggravate it as well.



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