Viral rash ?

My son broke out in a rash two weeks ago right before his 6 month check up.. the doctor said fine as long as he doesn’t have a fever and he’s acting normal.. she said it’s not an allergy because it goes away then comes back .. well last week he woke up this it so bad it was like a ring around his arm .. we were told it’s a viral rash and it should go away soon .. now today it’s on his other arm and legs and it’s freaking my out .. any ideas ? .. it starts off red then lightens but leaves bumps in that spot ..