Hi Y.,
Both my daughters had something like this. It turned out to be Roseola in both cases (something I also had when I was a baby). Unfortunately, my youngest daughter's pediatrician thought it was necessary to catheterize her and do blood work to rule out a very unlikely blood infection. It was a really traumatic day and if I had it to do over again, I'd just wait out the fever for a couple of days. Or at least would have chosen not to do such invasive stuff. :(
DEFINATELY take your little one in if you're concerned. The best advice I ever got was from my oldest daughter's pediatrician... she said, "trust your instincts". Follow what your gut tells you and you'll probably be just fine.
Just of note: If it IS Roseola, the rash does not bother them and you should not put anything on it except for tepid washcloths if you're trying to bring down the fever.
Hang in there. I know these things are super scary.
*hugs*
M.