Miscarriage after a Healthy First Child

Kelley
I delivered a healthy baby boy in April and had no complications during my pregnancy last year. 
Recently, on Christmas Day I found out I was pregnant again (took several tests) and today they said they believe I miscarried weeks ago and determined my uterus is empty and hormone levels are extremely low. 
The crazy thing is that I went to the ER for heavy bleeding and they believe it's actually my period starting again. How is this even possible this soon? 
I've only known I've been pregnant for exactly three weeks and now I'm going from excitement and planning to loss and feeling numb. 
Is there anyone who had a healthy baby, then miscarriage and healthy babies after their loss? If I had no problems getting pregnant and delivering a healthy baby last year, does mean it was possibly a fluke thing? 
I'm trying to process this all and going through a roller coaster of emotions. I'm trying to make some sense of this. I have an appt on Tues with the Dr. that delivered my first baby so I'm hoping he has some more answers or guidance for me.