It could be delayed pressured urticaria where pressure causes hiving on that part of the body. Just like the panty line where the elastics could have added pressure to your skin and that might be the cause of the rash. It would be good if you could go to your doctor and discuss about what I just wrote. There are info online about delayed pressure urticaria that you can get more understanding from.
New laundry detergent could also be the culprit. You might want to eliminate them for a period of time and see if you get better. At the moment, oatmeal baths or antihistamines might help with the rash and itch. Good luck!
See also dermatographism (sp), it's hiving after stroking the skin.