What Causes Red Splotches On Legs And Thorax After Starting BP Medications?

My wife is 54 years old and in pretty good health. Very mild high blood pressure but taking a pill for it. This condition started before she started taking the pill. Several nights a week she will break out in red sploches on her legs and buttox. The itch as well and feel warm to her. We thought it was the laundry soap but we have changes that several times. It almost always starts around 10:00 or so... What else can I give you??