A painful shoulder with a red skin may mean you need medical attention. However, the cause may be as simple as sunburn and may resolve on its own. Several other conditions that may not be relevant may cause these symptoms. Consult your doctor for a diagnosis and appropriate treatment.

Parvovirus infection
[ 123] Parvovirus infection is a highly contagious disease that usually affects children but may develop in adults. A bright red rash appears on the face, moving from the shoulder down to the arm or from the torso down to the hip is the farthest feature of this situation. Rash is rare in adults, but joint pain is common - although the shoulders are not as frequent as the wrists, ankles, knees and hands. Parvoviruses usually require very little treatment. The Mayo Clinic recommends that parvovirus infections become more severe for pregnant women, people with weaker immune systems and people with anemia.Sunburn
Severe sunburn can cause redness and pain in the skin. Shoulder or any place exposed to the sun. Your skin may become red, tender, or even warm. Red skin can develop into blisters in hours or days. The skin may peel off after a few days. According to the National Library of Medicine, chills, nausea, fever and rash may indicate sun poisoning, which is a serious reaction to sunburn.
Medication
Allergic reactions or side effects of taking medication can also explain your shoulder pain and red skin. For example, the drug lexapro may cause neck and shoulder pain as well as rashes, according to Drugs.com. This medication can cause some related musculoskeletal and dermatological side effects such as muscle stiffness, cramps, acne and dry skin. Ask your doctor if any medication you take will cause side effects.
Lyme disease
If you have mites on your shoulders, you may develop Lyme disease - also known as Borrelia or soil disease - - causing redness around the bite And pain. Symptoms described by NetDoctor include skin changes, jointsPain, muscle pain, lethargy, fever, swollen lymphadenopathy and headache. A red ring often appears around the pale spots on the bite; the redness may spread over time.
Hair rash