Demised baby i had that the doctor can't count for?

i had a baby when i was 16 going into my 5th mth i recived paper saying that my baby was demise but i heard the baby heart beat on a ultrosound but keep in mind i was at F.O.H for a regular checkup how is possible that something went wrong i really do believe the hospital did something with my baby because my other checkup said my baby heart beat was strong when i went to get the record there wasn't no record because i was never pregnant now tell me that was very,strange