A clinical examination is necessary for determining the cause of your symptoms. These red blotchy marks could possibly be seen due to an allergic reaction to any allergen. Red blotchy marks are usually seen in urticaria or dermatitis. Take bath with lukewarm water till your symptoms resolve. Take an antihistaminic drug like hydroxyzine for symptomatic relief. Application of calamine lotion may provide soothing effect. If the symptoms are severe or persistent visit a dermatologist. Keep me posted.
Best luck and take care!
These symptoms can be due to cholinergic urticaria. It is a form of chronic hives that is caused by an increase in body temperature but can be idiopathic also. Take some antihistaminic like Benadryl or Claritin and apply some calamine lotion on the hives. If the symptoms persist then please get it examined from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.