In my late teens, early twenties I would come up in big itchy lumps when I went out in the sun, not what I would call a rash. The GP then said it was solar dermatitis. It would gradually disappear the more I was in the sun and got a slight tan but it was horrible while it lasted.
The next time I came up in similar lumps I was diagnosed with scabies. The treatment did not work and they would appear intermittently for no apparent reason. I did not use any perfumed products and experimented by cutting out various food and drinks one by one. No links were made. The only helpful medication was sleeping tablets which at least allowed me to rest by knocking me out at night.
The last time I suffered was when I was pregnant. I remember getting up in the night and sitting in a cold water bath to ease the itching. It would more commonly appear at night and disappear the next day (though not always). They would be big red lumps but would go white when my skin was cold. The itching was unbearable.
Luckily it has now gone though over the years I have suffered various auto immune disorders, currently a form of alopecia. I still have no idea what caused the hives, as one doctor called them.