Welcome to HCM.
At your 20s, you had rash with fever. And rash would erythematous and itchy.
I feel, that you had viral exanthem
. ...most probably you had erythema infectiosum
.. Also called fifth disease
It is a viral disease caused by parvovirus B19.
There may be fever with erythematous rash
on face, trunk and extremities.
It is benign condition and heals without any complication.
Your present diseases fibromyalgia and RA are not related with your past disease.
I hope you got my answer.
Dr. Ilyas Patel MD