You seem to be having urticaria
. Sometimes urticarial rash develops at pressure area. so it can occur over hands, waist/ panty line or beneath bra straps. If urticaria is severe it can have other symptoms too like body ache
, headache, shivering, joint pain, abdominal pain
etc. to relieve the same you may continue taking Benadryl.
Please consult a dermatologist
nearby so that blood work can be done to diagnose the underlying cause if any.
Hope this helped.