I used to think that in POIS I have malaise (flu like symptoms). But I think it is a temporary urticaria. I also had 2 or 3 days of urticaria after using antibiotics for a few weeks. Now I have urticaria for almost 5 months. And that feels like POIS. Anti histamine did not help me against urticaria. I already used 5 types of anti histamine. I am not saying that POIS=urticaria. But I think urticaria is 1 of the symptoms of POIS, at least in my case. And pain in muscles and joints is part of urticaria.