Please help me.

Everytime {for the past 3 years} I get a cold of flu I get a horrible rash all on my body. Im talking lots of rash in lots of places. Under my breast along my ribs around my hips all along my forearms and around my armpits and along my neck and on my buttocks. It itches and is very bright. Sometimes when it gets really bad I will get welps on certain areas. This happens everytime I get sick. No one can tell me why it happens and if it's dangerous. Im guessing it's not dangerous because it has never cause problems other than annoyance. Has anyone else ever had this problem or heard of this? Oh and it's not contagious at all. My boyfriend has never got it. It will go away after a few days of being well again.
S.