What Causes Red Welts On The Arms, Legs And Back?

hello, my 26 year old daughter has some red welts on her arms, legs, stomach, back and each day there seem to get a little more. They don t itch or are raised. They are just red welts spreading. She is white, and she hasn t changed her eating habits, detergent. Says they aren t bites. No one in her apt. has anything on them. Her fiance doesn t have anything on him either. She doesn t have pets. She thinks it could be stress related. She s a little anxious about it. What do you think? my email is YYYY@YYYY