Assuming your wife's rash is hives, the most common cause would be a reaction to a medication, including vitamins or any supplement. Ibuprofen (Motrin, Advil) or Aleve are the most common over-the-counter medications that cause allergic reactions. Another potential cause would be a reaction to a food or food additive. This reaction would likely start within a few hours of ingesting an offending food and would almost always resolve within 24 hours.
Frequently hives can be associated with an overreaction of the immune system to any type of infection, including colds, flu, viruses, skin infections, bladder infections, etc. If there are symptoms in any other part of her body, the possibility of these infections needs to be investigated.
If hives continue for more than one to two months, they can be associated with the onset of other medical conditions, especially auto-immune diseases like thyroid or arthritis conditions.
Once hives occur, they are aggravated by heat, dry skin, stress, and by scratching. Over-the-counter antihistamines are effective at minimizing symptoms. For daytime use, loratadine (Claritin) or cetirizine (Zyrtec) work well, with diphenhydramine (Benadryl) the best nighttime option. If these medicines do not help, or are needed for more than a few days, be sure to see a doctor.