I cannot confirm anything without examination,but it can be due to solar dermatitis or heat rash. It is the development of hives minutes after exposure to the sun, which resolve in less than 1 hour.
Apply a good sunscreen with minimum spf 30 atleast half an hour before going out in the sun.You can take non-sedating antihistamines such as Cetirizine or Loratadine(Claritin) when the rash appears. Short courses of oral cortisone or steroids are taken for short periods (one to three days) to settle more severe symptoms.You may take Vitamin C along with the prescribed treatment. Vitamin C is a general anti-allergy supplement. Also apply calamine lotion and a mild steroid cream like dermacort on the rash. Also use cotton clothes and change the clothes twice if there is any increased sweating.
If the symptoms persist then pls consult a dermatologist.
I sincerely hope that helps. Take care and keep me posted Kind Regards.
I already take two antihistamines and 1000 mg of vitamin c a day.