Thanks for posting your querry on HealthcareMagic.
After reading your symptoms, i feel that it is aome kind of an allergic reaction
as it is not limited to just shoulders. I would recommend you to take any anti allergic medication for 2-3 days as it may solve your problem. You can also apply calamine
lotion topically in the areas of the red marks and aloe vira gel for symptomatic relief.
If your condition does not improve then you must visit your dermatologist
and undergo allergy tests
or any other tests he deems necessary.
Wish you a speedy recovery.