Miscarriage :(

Found out I was pregnant on Saturday morning, April 12th. So excited, not planned, but excited none the less. Tuesday morning, the 15th, started bleeding very heavily, went to ER got checked out, they said threatened misscarriage. Went to doc Thursday, got blood checked, hormone levels waaaay down. Diognosed as miscarriage. We already have a boy that turned 4 on April 9th, delivered by emergency c section after hours of labor( he was too big for me to deliver naturally). But had a completely normal pregnancy. I'm only 23 years old. Some thoughts on what may have caused this? And my main question is when can we try again?