What Causes Red/violet Rashes In Body?

my 13 year old son has beenon adderall for over a year . he was changed to vyvanse a month ago. he is behind in school and has an overall dislike to going. He has had head and chest congestion for several days. he has woke up with a flat red rash on most of his body for 3 days. by mid day it is gone. on the 2nd day he was at school and had a violent looking rash. picked him up from school. 3 hours later - completely clear. the er doctor did not know what it is.