My 3yo hasn't been herself. Pale, low grade fever, rash on her neck & cheek & now she says her tummy hurts.?

she's been eating normal, normal bms. Just not her usual sunny self. Got worried, & I'm normally not a worrier. It just doesn't feel right. I know the doc is just gonna say it's a virus & let it run the coarse. Anyone experienced these symptoms in their kids?