As you had the rash,soreness and itching which began immediately after anal sex,it could be because of
friction rub due to non lubrication and relaxation or irritant contact dermatitis.
Anal tissue is delicate and easy to damage and so if there is any intercourse without adequate lubrication and
relaxation,it can cause discomfort and itchy rashes.
Pls abstain from anal sex for some days and use soft tissue to clean your rear or wash it with water.Pls
remember to dry the area after washing by pat drying.Try taking some over the counter antihistaminic like
benadryl or Claritin for the itching.Apply some calamine lotion or mild corticosteroid cream on the rashes.If
the symptoms persist then it is important to rule out pinworms or sexually transmitted infections.Pls consult
a gynecologist in that case.
Hope it helps.Take care and pls do keep me posted if you have any additional queries.