It could be an allergy to a material in the clothes (polyester, rayon) or to a detergent you've been using. I had this as a baby and a doctor suggested cornstarch and washing my clothes separately with a gentle detergent (try Tide Free or Dreft).
Later on my skin became very sensitive to heat and any type of soap. One doctor prescribed an anti-fungal creme one summer (?!) which was a little bit of overkill. To treat the rash (later diagnosed as PUPPP when I was pregnant) I do make one exception - PUPPP Relief Soap from Rainier Soapworks, sold online. It's organic, non toxic and was incredibly effective at getting rid of the red bumpy rash and did not cause breakouts. They have a PUPPP Relief™ Cream as well. It's normally for pregnancy but it has helped me at other times too, and it smells terrific. Here are before & after pics:
http://www.rainiersoapworks.com/puppp_relief.htm
Other than that, I usually use the yellow Johnson & Johnson's Baby Wash all over (including my face) and cool cotton clothing (Gap is good).