I am 16 week pregnant based on my ultrasound last march and ba ys heart beat was 133

Follow up check up ko last april9 and my babys heart beat was 134. But yesterday check up ko ulit and it breaks my heart nung hindi mahanap ung heartbeat ni baby.so dr recommended me to take another ultrasound and the impression was walang baby sa uterus ko...i cant understand how and why this things happened hindi naman ako nagbleeding since april...