8 mth old just saw familly doc on 3 days ago. had no sign of ear infection, or cold. started fever today of 39.2 , no other symptoms. what could itbe?

Could be many things: Kids can go from healthy to a cold with full blown ear infection in < 24hrs. I've seen it happen in less than 8 hrs in a 5 yo. Seeing the doc one day does not give anyone at any age a one day, week or month pass on beginning an illness that was not yet in play earlier.If the physical symptoms change, the child should be re-examined based on that change.