My son had a virus like that- our doctor may have said roseola but I remember "viral measles" which was his way of explaining to us that it was *like* measles but it wasn't as it's a virus. He was about the same age and we had delayed the MMR vax too so we did get raised eyebrows when the doctor first saw it and asked if he was vaccinated. One thing I wish I'd thought to do was warn the doctor's office so they can ensure you are isolated from other patients when waiting, just in case it is rubella and there is someone else there who is pregnant, a young baby etc. We just went to the doctor as normal and I felt so awful when I realised later what we had done.
Good luck- I'm sure she'll be fine