There is no particular medical reason that I can think of that would cause the shoulders to burn more rapidly than any other part of the body, assuming equal sun exposure.
Therefore, I think it is more likely that your shoulders are burning more rapidly than other parts of your body because they are getting more direct sun exposure! For example, if you are standing up or walking around outside, the sun will directly strike your shoulders but only indirectly strike your legs or your stomach - resulting in more rapid burning. At the same time, the shoulders are harder to evenly apply sunscreen to than are the legs or stomach as you cannot see them as well, and this could also be contributing to your sunburn.
I would recommend talking to your primary care doctor
or a dermatologist
to see if they have any recommendations about specific sunscreen products. Also, they will be able to give you general advice about protecting your skin from sun damage, which is one of the major causes of accelerating skin aging and also a risk factor for the development of skin cancer.
Please talk to your doctor!