Guys I'm confused!?

Crystal
I was told the first week of spring semester that I was having a miscarriage. I was crushed and confused, the pregnancy was a surprise and my first.  To go through the difficult process of accepting an unplanned pregnancy, falling in love with my unborn child, and then being told I lost it was awful. 
But the Universe is always full of surprises.
I never got a period after my miscarriage and the doctors have yet to tell me exactly what happened. I am 15 weeks pregnant according to size estimates from the last ultrasound I got at 11 weeks. 
My nurse told me that it may even be the same baby and maybe I didn't have a miscarriage after all! Or I got pregnant immediately after the miscarriage. Either way I am so happy to have my little one with me!
Does anyone have any ideas of what may have happened? I am so happy I am pregnant, but I don't understand how. Ideas anyone?