rash lasting about 5-10 minutes

My daughter is 11.  She has been getting a rash on her stomach and hands.  It occurs at odd times and I haven't been able to relate it to food.  I thought that it might be a new fabric softner but that  would not explain it on her hands.  It only last between 5 and 10 minutes but is painful while it is there.  It just started about a month ago and seems to be growing in frequency.  I called the doctor here and they said I should bring her in when it happens so he can look at it, but with school and such that is not as easy as it sounds.  Please anything you can tell me will help.  
deb